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Obsessive-compulsive disorder is a mental illness that involves repetitive, ritualistic behaviour. Some of those with the condition wash their hands every few minutes, frantically trying to rid themselves of germs, while others become locked in cycles of obsessively checking they have turned off the stove.

Various causes have been proposed, ranging from genes to traumatic childhoods. More recently, interest has focused on a part of the brain called the basal ganglia, thought to be involved in decision making. Cases of OCD have arisen after injury to this area, for example, after a stroke or blow to the head.

A small subset of cases, however, may have an infectious origin. In the 1990s, Susan Swedo, a neuroscientist at the US National Institutes of 午夜福利1000集合 in Bethesda, Maryland, noted that a few children newly diagnosed with OCD had recently been infected by streptococcus bacterium, the cause of many a sore throat.

Streptococcus was already known to cause other autoimmune diseases, such as rheumatic fever, in which molecules involved in the immune attack against the microbe cross-react with molecules from the human host. Swedo speculated that in this case the human molecules involved are in the basal ganglia or other circuits of the brain involved in OCD.

Swedo鈥檚 group coined the term PANDAS, for paediatric autoimmune neuropsychiatric disorders associated with streptococcus infections, to describe the condition. Some small studies have found that children with suspected PANDAS have antibodies in their blood to certain proteins found in the brain, although other researchers have not found such antibodies. There have also been a few suspected cases in adults.

The bacterial connection with OCD is still being debated, and a recent large study in Neurology found no link. But Swedo and others are already using it as a basis for new treatments aimed at people who do not respond to the standard treatment of therapy and antidepressants.

Several techniques to rein in the autoimmune attack have been trialled so far, including filtering antibodies out of the blood, and antibiotics to prevent repeated streptococcus infections. A few small studies have shown success but other trials have not, and the approach remains controversial.

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Read more: Six diseases you never knew you could catch