The exact cause of Behçet's disease is unknown, although it is generally thought to occur as a result of a problem with the immune system.

Most experts consider Behçet's disease to be an autoinflammatory condition. This means a condition where certain components of the immune system (the body's natural defence against infection and illness) mistakenly attack healthy areas of the body.

Normally your immune system will cause inflammation (swelling) in areas of the body that have become infected.

But in Behçet's disease, the inflammation affects parts of the body where there is no infection, probably starting in the lining of blood vessels.

This is why there is such a wide range of symptoms – any part of the body that has a blood supply can be affected.

It's not clear what triggers this problem with the immune system in people with Behçet's disease, but it has been suggested that both genes and environmental factors are likely to be involved.

Genes 

One theory is that some people are born with genes that make them more vulnerable to developing Behçet's disease.

This is because Behçet's disease tends to be much more common in certain ethnic groups where the genes that are linked to the condition may be more common.

For example, the condition is many times more common in Turkey than in northern Europe. Having a family member with the condition may also increase your risk of developing it.

Research has so far shown that many people who have developed Behçet's disease have a gene called HLA B51, and this may increase the risk of developing the disease considerably. Variations in other genes, such as IL10 and IL23R-IL12RB2, are also more common in people with the condition.

Possible environmental factors

It's thought genetics are not solely responsible for Behçet's disease as there are many cases of people who have the condition but don't have any of the genes associated with it.

Ethnic groups known to be at risk of developing the condition can also reduce their risk by leaving their native country. For example, rates of Behçet's disease are lower in Turkish people who live outside Turkey. This suggests that an external environmental factor may also have a role in triggering the condition.

As yet, a specific environmental factor involved in the development of Behçet's disease has not been identified. However, it is often suggested that a viral or bacterial infection may play a role, or that the range of bacteria (the microbiome) present in individuals with Behçet's disease is somehow different.

The theory is that when a person comes into contact with this virus or bacteria, something in their genes causes their body to react badly and attack their healthy tissue, leading to Behçet's disease.