Most cases of glandular fever are caused by the Epstein-Barr virus (EBV).
The Epstein-Barr virus
If you come into close contact with infected saliva and are not immune (resistant) to glandular fever, EBV will infect cells on the lining inside your throat.
It can be caught by close personal contact, especially kissing (it's sometimes referred to as the "kissing disease"). It may also be possible to catch it from sharing toothbrushes and drinking utensils.
The infection is then passed to white blood cells before spreading through the lymphatic system. This is a series of glands (nodes) that spread throughout your body in a similar way to your blood circulation system. The glands produce many of the cells that are needed by your immune system.
The spleen is an important part of the lymphatic system, because it helps produce the infection-fighting antibodies your immune system uses to fight infection. If your spleen is infected, it will become inflamed (swollen). This occurs in around half of all cases of glandular fever.
It is unclear exactly why some people develop the symptoms of glandular fever after coming into contact with EBV, while others do not. Age appears to be the most important factor, as most cases affect older teenagers and young adults.
There is also evidence that some people may be born with certain genes that make them more susceptible to developing glandular fever.
A few cases of glandular fever are caused by viruses other than EBV, such as:
Toxoplasmosis is a parasitic infection that can also cause similar symptoms to glandular fever.
Other causes of glandular fever are usually only a matter of concern for pregnant women. This is because unlike EBV, other viruses can harm unborn babies. Additional treatment with antiviralmedication (special antibodies) and antibiotics may be required to reduce the risk to your unborn baby.
An early HIV infection can also cause symptoms of glandular fever. Inform your GP if you think you may have been exposed to HIV infection in the previous two months.
Your GP will be able to carry out a blood test to check for HIV infection. If you have HIV, it is very important it is diagnosed at an early stage, as excellent treatments for the condition are now available which may be of benefit during the early stages of the infection.