Doctors know surprisingly little about the causes of encephalitis. Outbreaks of encephalitis in a community are usually linked to insect-borne viruses, like the West Nile virus carried by mosquitoes that has sickened more than 50,000 people since it was first detected in the United States in 1999.
But many more encephalitis cases are isolated, and usually even the most diligent attempts to pinpoint a cause fail.
Viruses and bacteria are believed to account for most cases, but a specific germ is positively identified in fewer than half of all cases.
The virus gets into the body through insect bites, skin contact, or in food or drink. The viruses may be carried by mosquitoes or ticks (see arboviruses), especially in rural areas. In urban areas, other types of viruses, usually enteroviruses (see below), may be responsible.
Rabies, passed to people when they are bitten or scratched by an infected animal, causes a deadly form of encephalitis, but is exceedingly rare.
Better known viruses like measles, mumps, rubella, and herpes may also cause encephalitis.
In post-infectious encephalomyelitis, the body apparently overacts to more common infections, like measles (measles encephalitis) and the flu, attacking protective coatings around brain and nerve cells. It has been likened to an "allergic reaction" to an earlier infection. Fortunately it is rare. Measles encephalitis can be particularly severe.
While there are more than 100 germs known to cause encephalitis, some of the more common are:
Enteroviruses, which live in the gut and are and spread via feces. Most of these cases are mild with no lasting effects. However, death can occur in rare, very severe cases. They are most likely to appear in summer and fall. They include echovirus, coxsackievirus, and many other viruses.
Herpes Simplex Virus, type 1 and type 2. These viruses are responsible for more serious cases. HSV type 2 (genital herpes) is usually passed from a mother to her newborn during delivery. HSV type 1 (which causes cold sores and is carried by most adults) lies dormant in most people, but can become activated and travel to the brain by an infection or another trigger.
Arboviruses, which are viruses spread by insects, can cause St. Louis encephalitis, Western equine encephalitis and Eastern equine encephalitis. The most common is California encephalitis, particularly the strain called La Crosse, which is found mostly in the Midwest and eastern United States. Arboviruses are most likely to appear in summer and fall.
Bacterial causes of encephalitis include mycobacterium tuberculosis (TB) and Bartonella henselae, also known as cat scratch disease because that is how it is usually transmitted.
More rarely, encephalitis can be triggered by a brain injury, a brain tumor, drug reactions, or a poison (e.g., lead poisoning), or by as a reaction to a vaccine.