About one in 600 newborn boys will have the genetic condition Klinefelter syndrome (sometimes called Klinefelter’s or XXY), which means they are born with an extra X chromosome.

This will have happened randomly during the formation of the egg or sperm, or after the baby boy was conceived. The extra chromosome is not inherited.

Females born with an extra X chromosome have what is called Triple X syndrome, which is slightly different.

Adult males with Klinefelter syndrome may not be able to produce enough testosterone, the sex hormone responsible for the development of male characteristics and important for maintaining bone strength, libido and fertility in men.

The features that result from this – such as low energy and sex drive, lack of muscle and too much body fat around the middle – may not be noticeable until after the age of puberty.

Most men with Klinefelter syndrome live independently and have normal lives, jobs and relationships, and will be unaware of their chromosome variation. Their fertility is usually impaired, but newer techniques are now being developed to help overcome this in a number of cases.

Infants

Klinefelter syndrome is hard to spot in infants, as some of the more common features may not be present or obvious at first.

Babies and toddlers with Klinefelter syndrome may:

• be born with undescended testicles 
• be slow to sit up, crawl and walk
• not start talking until later than average
• have low muscle power 
• be quiet and passive

Boys and teenagers

Some boys with Klinefelter syndrome can be shy with low self-confidence. About two-thirds may have problems at school with reading, writing, spelling and paying attention. They may also have: 

• low energy levels
• mild learning disabilities such as dyspraxia and dyslexia 
• difficulty socialising or expressing their feelings

Boys may grow more quickly than usual during childhood and become taller than expected for the family. Most of the extra growth is in the legs, and the hips may get broader too. Extreme tallness is unusual and can be predicted early on in childhood.

Puberty starts at the normal time (11-13 years) with testicle growth as in all boys, but the testicle size does not increase as usual.

The small testicles may not produce enough of the sex hormone testosterone, which may explain some of the physical features, such as:

• a flabby body with low muscle tone
• a reduced amount of facial and body hair
• small, firm testicles and a small penis
• enlarged breasts (gynaecomastia; affects about two-thirds)
• reduced calcium in the bones

The lack of testosterone may delay the completion of sexual development during puberty.

Even at this later life stage, these signs of Klinefelter syndrome may not be noticeable and the condition may not be diagnosed until adulthood, when the GP is consulted because of fertility problems.

Advice at this stage

See your GP if your son has slow development for his age, especially if you recognise some of the signs listed above.

It’s worth diagnosing any sort of growth or developmental problem early, as treatment can prevent or improve some of the outcomes. See What treatments are available? for more information.

Participation in sport will help build muscle strength and independence.

You might find it helpful to connect with other parents of children with Klinefelter syndrome. Find out more in More information and support. 

Men

Men with Klinefelter syndrome may produce lower amounts of testosterone than usual and will not generally be able to conceive naturally as the testicles produce little or no sperm.

New techniques are being developed to extract sperm from the testicles to help with artificial conception methods, which may help in some circumstances.

If a man with Klinefelter syndrome was not diagnosed and treated at the time of puberty, he may have some or all of the following features:

• a low sex drive
• taller than average for the family 
• reduced amounts of facial and body hair
• belly fat and looser muscles
• enlarged breasts (gynaecomastia) 
• small testicles and sometimes also a small adult-sized penis

Anxiety, learning difficulties and depression are common, although intelligence is usually unaffected.

Despite these problems, most men with Klinefelter syndrome will have normal independent lives, holding down relationships and jobs.

It may be helpful to talk to your doctor about treatments and discuss family planning options. The useful links on this page list support groups, where links to other men with Klinefelter syndrome can be made.

Possible complications

Men with Klinefelter syndrome are generally healthy, but at slightly greater risk than average of:

• varicose veins and other blood vessel problems
• lung disease
• autoimmune disorders (where the immune system attacks the body’s own tissues) such as type 1 diabetes and lupus

What’s the cause?

A male with Klinefelter syndrome is born with an extra X sex chromosome in his cells – XXY, rather than the usual XY (see the box on this page for an explanation of sex chromosomes).

This second X chromosome carries extra copies of genes, which interferes with the development of the testicles.

Some males will have a milder form of Klinefelter syndrome, where they only have XXY chromosomes in some of their cells, rather than all of them. This is known as mosaic Klinefelter syndrome.

How is Klinefelter syndrome diagnosed?

GPs may suspect Klinefelter syndrome after physically examining their patient’s genitals and chest and testing a blood or urine sample for the presence of abnormal hormone levels.

The diagnosis can be confirmed after ordering a genetic test, which involves sending a blood sample to a laboratory to check if there is an extra X chromosome (chromosomes can be seen under a powerful microscope).

What treatments are available?

Klinefelter syndrome cannot be cured as there’s no way to fix the extra chromosome, but treatments and therapy can help, including:

• testosterone replacement therapy
• speech and language therapy
• educational support (through educational psychology)
• emotional and behavioural support (through psychology)
• physiotherapy
• counselling to work through emotional issues and infertility
• infertility treatment (IVF)
• surgery to remove excess breast tissue

Testosterone replacement therapy (TRT) 

TRT involves being prescribed a medication containing a hormone similar to testosterone.

It can be taken in the form of gels or patches, or given as injections to correct the hormone deficiency.

It cannot reverse infertility, but can treat and prevent many of the typical features of Klinefelter syndrome.

Ideally, an assessment should be made at the time of normal puberty, around the age of 12. Even if it is not needed, a repeat assessment should be made in the mid- to late-teens, as a deficiency may show up only at that stage onwards. 

If needed, TRT helps with normal development during puberty, such as a deep voice, facial and body hair, and an increase in muscle mass (although it won’t cause the testicles to grow). It should also improve bone density and reduce the risk of fractures.

Referral to a paediatric or adult endocrinologist (specialist in hormone disorders) according to age is necessary. The specialist can make the appropriate assessments. 

More information and support

Klinefelter’s Syndrome Association UK

Klinefelter Organisation

Contact A Family

Yellow fever is caused by a type of virus known as a flavivirus. The infection is transmitted by the bite of certain mosquitoes.

Yellow fever cannot be spread by close contact between two people.

How yellow fever is spread 

The Aedes aegypti mosquito usually spreads yellow fever in urban and some rural areas. However, in forested areas, other types of mosquito may also carry the virus. The mosquitoes usually bite during daylight hours.

The mosquito becomes infected by biting a monkey or human who is already infected with the virus. Infected mosquitoes can then pass the disease on to other monkeys or humans that they bite. Once infected, a mosquito is a source of infection throughout its life.

The virus is thought to be widespread among monkeys that live in the jungle canopy (the tree tops) of some parts of Africa and the Americas.

Occasionally, an infected mosquito will pass the virus on to a person in the jungle, such as a forestry worker, who may then become a source of infection when they return to their community.

The risk of an urban yellow fever outbreak is highest in areas close to jungles where the mosquitoes and infected monkeys live.

Am I at risk?

If you are travelling, your risk of getting yellow fever will depend on:

• where you are travelling to and whether there is currently an outbreak of yellow fever in the area (see risk areas for yellow fever)
• whether you have been vaccinated for yellow fever
• whether you are visiting jungle or forested areas

Yellow fever is usually diagnosed based on the symptoms and the results of a blood test.

It can sometimes be difficult to diagnose yellow fever based on the symptoms alone because they are often similar to those of other conditions such as:

• malaria – a tropical disease that is spread by night-biting mosquitoes
• typhoid fever – a serious and potentially fatal bacterial infection
• viral hepatitis – inflammation of the liver caused by a virus
• leptospirosis – a bacterial infection spread to humans by animals, such as rats
• Dengue fever – an infectious disease that, like yellow fever, is transmitted by the Aedes aegypti mosquito

Blood test

If you have yellow fever, a blood test will be able to detect the presence of special proteins called antibodies, which are produced by the body to fight the virus.

The blood test may also show a reduction in the number of infection-fighting white blood cells (leukopenia). This can occur because the yellow fever virus affects your bone marrow (the spongy material at the centre of some bones that produces blood cells).

Yellow fever is a serious viral infection that’s usually spread by a type of mosquito known as the Aedes aegypti mosquito. It can be prevented with a vaccination (see below).

Yellow fever mainly occurs in sub-Saharan Africa (countries to the south of the Sahara desert), South America and in parts of the Caribbean.

There have not been any recent cases of yellow fever in North America, Europe or Asia. Since 1996, six travellers from Europe and North America have died from the infection. None of them were vaccinated. 

Read more about the risk areas for yellow fever.

Yellow fever can be fatal. About eight people out of 100 who get yellow fever die from it.

Yellow fever symptoms

Typical symptoms of yellow fever include:

• high temperature (fever)
• headache 
• nausea and vomiting
• muscle pain, including backache
• jaundice – yellowing of the skin and the whites of the eyes caused by liver damage

How yellow fever is spread

Yellow fever is a type of virus known as a flavivirus. The virus is transmitted through mosquito bites. It can be spread from one host to another, usually between monkeys, or from monkeys to humans, or from person to person. The mosquitoes live and breed in jungle and urban areas.

Read more about how yellow fever is transmitted.

Diagnosing yellow fever

Yellow fever is usually diagnosed based on the symptoms and a blood test.

It’s difficult to diagnose yellow fever based on the symptoms alone because they are often similar to those of other conditions, such as malaria and typhoid fever.

Read more about diagnosing yellow fever.

Treating yellow fever

There is no specific antiviral treatment for yellow fever, but the symptoms can be treated.

Headache, high temperature and muscle pain can be treated using painkillers, such as paracetamol or ibuprofen. You should also drink plenty of fluids to avoid dehydration.

If your symptoms are severe, you may be admitted to hospital so that your condition can be monitored and you can receive supportive nursing care.

Read more about treating yellow fever.

Yellow fever vaccination

There is a vaccination for yellow fever. Some countries require proof of vaccination (a certificate) against yellow fever before they let you enter the country.

You should have a yellow fever vaccination at least 10 days before your travel. This will allow enough time for your body to develop protection against the yellow fever infection.

The yellow fever vaccination is recommended for:

• laboratory workers who handle infected material
• anyone travelling to, or living in, areas or countries where yellow fever is a problem
• anyone travelling to a country where an International Certificate of Vaccination or Prophylaxis (ICVP) against yellow fever is required for entry

Read more about the yellow fever vaccination.

Preventing mosquito bites

As well as getting the yellow fever vaccination before travelling, you should also take steps to avoid being bitten by mosquitoes.

The mosquitoes that carry yellow fever bite during daylight hours. Although it may not always be possible, you should try to:

• avoid places where mosquitoes live, such as swamps, forests and jungles
• choose air-conditioned accommodation
• choose accommodation with mesh screening over the windows and doors
• wear loose fitting, long-sleeved tops and trousers
• use insect repellent containing DEET on exposed skin
• burn a mosquito coil or use a plug-in device that releases insecticide

The World Health Organization (WHO) keeps an up-to-date list of the countries and regions where there is an increased risk of yellow fever.

Although the number of yellow fever cases in some countries may be low, visitors may still be at risk of infection. For example, low rates of yellow fever could be due to a vaccination programme in the area. However, there may still be a risk of unvaccinated visitors becoming infected.

Read more about the yellow fever vaccination.

Risk areas in Africa

Most cases of yellow fever occur in sub-Saharan Africa (the countries and regions south of the Sahara desert). In the past, other areas of Africa have also been affected by outbreaks of urban yellow fever.

The areas of Africa where there is a risk of getting yellow fever are:

• Angola
• Benin
• Burkina Faso
• Burundi
• Cameroon
• Central African Republic
• Chad
• Congo
• Ivory Coast (Côte d’Ivoire)
• Democratic Republic of the Congo
• Equatorial Guinea
• Ethiopia
• Gabon
• Gambia
• Ghana
• Guinea
• Guinea Bissau
• Kenya
• Liberia
• Mali
• Mauritania
• Niger
• Nigeria
• Rwanda
• Senegal
• Sierra Leone
• South Sudan 
• Sudan 
• Togo
• Uganda

Risk areas in South America

The areas of South America where there is a risk of getting yellow fever are: 

• Bolivia
• Brazil
• Colombia
• Ecuador
• French Guiana
• Guyana
• Panama
• Paraguay
• Peru
• Suriname
• Venezuela

See the Fit for Travel website for maps showing the yellow fever risk areas in Africa and South America.

Risk areas in the Caribbean

In the Caribbean, there is a risk of getting yellow fever in Trinidad.

After being infected with yellow fever, it usually takes three to six days for the symptoms to appear. This is known as the incubation period.

The symptoms of yellow fever can occur in two distinct stages.

First stage

The symptoms of the first stage, also known as the ‘acute phase’, include:

• a high temperature (fever) of 38ºC (100.4ºF) or above
• chills (shivers)
• headache
• nausea and vomiting
• muscle pain, including backache 
• loss of appetite

Most people improve after three to four days and their symptoms disappear. However, some people go on to develop more serious symptoms.

Second stage

After the initial symptoms of yellow fever, around 15% of people develop more severe symptoms. This is sometimes referred to as the ‘toxic phase’. The symptoms can include: 

• a recurrent fever 
• abdominal pain
• vomiting
• jaundice – a yellow tinge to the skin and whites of the eyes caused by liver damage
• kidney failure 
• bleeding from the mouth, nose, eyes or stomach, leading to blood in your vomit and stools (faeces)

Half of those who progress to the second, toxic phase of yellow fever die within 10-14 days. The other half recover with no major organ damage and are immune from the condition for the rest of their life.

Overall, this means about seven or eight people out of every 100 who develop yellow fever will die from it.

There is no specific antiviral treatment for yellow fever. However, the symptoms can be treated.

A high temperature (fever), headaches and back pain  can be treated using painkillers, such as paracetamol or ibuprofen.  

You should also drink plenty of fluids to avoid dehydration.

Hospital treatment 

If your yellow fever symptoms are severe, you may be admitted to hospital so you can be monitored and receive supportive nursing care. Additional treatment may be necessary, including:

• a ventilator to help you breathe
• a blood transfusion – of red blood cells to replace those lost through bleeding
• dialysis – where a machine is used to filter your blood if your kidneys are no longer working

Vaccination is the most effective way of preventing yellow fever.

In the UK, Stamaril (produced by Sanofi Pasteur MSD) is the only licensed yellow fever vaccination. A single dose of the yellow fever vaccine will protect against yellow fever for at least 10 years. It is recommended that you have a booster dose every 10 years if you are still at risk of infection.

Who should be vaccinated?

You should have the yellow fever vaccination if you are:

• a laboratory worker and handle infected material
• travelling to a country where you need an International Certificate of Vaccination or Prophylaxis (ICVP) before being allowed into the country – the certificate proves that you have been vaccinated against yellow fever
• travelling to, or living in, an area or country where yellow fever is found (see risk areas for yellow fever for a list of these countries)

You must have a yellow fever vaccination at least 10 days before you travel. This will allow enough time for your body to develop protection against the yellow fever infection.

Your certificate will only become valid 10 days after you have the yellow fever vaccination.

Where do I get vaccinated?

Yellow fever vaccinations can only be given at designated and registered centres. For a centre to become a designated yellow fever vaccination centre, it must register with the appropriate authority. In the UK, this is either:

• the National Travel Health Network and Centre (NaTHNaC)
• Health Protection Scotland (HPS)

Find your nearest yellow fever vaccination centre.

Certificate of proof

Under regulations set out by the World Health Organization (WHO), anyone travelling to a country or area where the Aedes aegypti mosquito is found must have an International Certificate of Vaccination or Prophylaxis (ICVP).

You can find a list of all the countries that require you to have an ICVP in the WHO International travel and health guide. You can also search the country information on NaTHNaC to find out whether the places you are visiting require an ICVP. 

If you have been travelling in an “at-risk” area during the past month, it is a good idea to carry your certificate with you. This will help avoid potential problems with immigration. It is possible for travellers without a valid yellow fever vaccination certificate to be vaccinated and held in isolation for up to 10 days. An ICVP is not required for entry into the UK.

If you lose your certificate, you may be able to get another one reissued as long as you have details of the vaccination batch number and the date you had the vaccination.

Seeking medical advice

Always consult staff at a designated vaccination centre if you are planning to travel to an area where there is a risk of getting yellow fever. If you tell them where you are travelling to, they will be able to advise you about whether you need to be vaccinated against yellow fever and whether you need an ICVP.

Who should not be vaccinated?

People who should not have the yellow fever vaccination include:

• babies under nine months of age – babies who are six to nine months old should only be vaccinated if the risk of getting yellow fever during travel is unavoidable
• pregnant women – unless the risk of yellow fever is unavoidable
• breastfeeding women – unless the risk of yellow fever is unavoidable
• people whose immune systems are lowered (immunosuppressed) – such as people with HIV and those receiving chemotherapy or radiotherapy
• people who are allergic to eggs – the vaccine contains small amounts of egg
• people who have had a severe allergic reaction (anaphylaxis) to a previous dose of the yellow fever vaccine
• people who are allergic to any of the ingredients in the vaccine
• people who have a condition that affects the thymus gland (part of your immune system that is located in your upper chest)
• people who are currently very unwell (such as with a high fever) – this is to avoid confusing the diagnosis of your current illness with any side effects from the vaccine
• yellow fever naïve travellers – those who have not been previously exposed to the vaccine who are 60 years of age or over (unless the risk of yellow fever is unavoidable)

Exemption letters

In cases where having a yellow fever vaccination is not advised, your GP may be able to issue you with an exemption letter. The letter should be written on headed notepaper and include the practice details. It may be accepted by some immigration authorities.

If you are travelling from an area where there is a risk of yellow fever without a valid yellow fever certificate, immigration officials are legally entitled to quarantine you for a period of at least seven days at the point of arrival into a country.

Side effects of the vaccine

After having the yellow fever vaccine, 10-30% of people will have mild side effects such as:

• headache 
• muscle pain
• soreness at the injection site
• mild fever

Reactions at the injection site usually occur one to five days after being vaccinated, although other side effects may last for up to two weeks.

An allergic reaction to the vaccine occurs in one case out of every 130,000 doses of the vaccine that are given. 

Yellow fever vaccine-associated neurological disease (YEL-AND)

Rarely, the yellow fever vaccine is associated with a neurological condition known as yellow fever vaccine-associated neurological disease (YEL-AND). Neurological means that it affects the nerves and the nervous system, including the brain and spinal cord.

YEL-AND occurs in around four cases out of every 1 million doses given. However, for people who are 60 years of age or over and yellow fever vaccine naïve, the incidence of YEL-AND increases to around one in every 50,000. This represents the highest risk for any vaccine currently in use.

The symptoms of YEL-AND include:

• a high temperature (fever) of 38ºC (100.4ºF) or above
• headache
• confusion
• problems with your nerves – for example, a problem with the nerves in your tongue that affects your ability to speak (focal neurological deficit)
• coma (a state of unconsciousness caused by injury or illness)
• Guillain-Barré syndrome (inflammation of the network of nerves that control the body’s senses and movements)  

Yellow fever vaccine-associated viscerotropic disease (YEL-AVD)

The yellow fever vaccine is also associated with yellow fever vaccine-associated viscerotropic disease (YEL-AVD). Viscerotropic means that it affects the viscera – your internal organs, such as the heart or lungs.

YEL-AVD occurs in around three cases out of every 1 million vaccines that are given. However, for people who are 60 years of age or over and yellow fever vaccine naïve, the incidence of YEL-AVD increases to just over one in every 50,000. This represents the highest risk for any vaccine currently in use.

Symptoms of YEL-AVD include:

• a high temperature (fever) of 38ºC (100.4ºF) or above
• headache
• muscle pain
• hepatitis (inflammation of the liver)
• hypotension (low blood pressure) 
• multiple organ failure