Meningitis Click for a larger view and caption

Microorganisms that infect people and cause illness are called pathogens. Some pathogens can cause disease by prompting a serious inflammatory response in a particular organ, such as the brain. Two well-known examples of infections that cause inflammation in the brain are meningitis and encephalitis.

Meningitis is a serious inflammatory condition resulting from bacterial or viral infection of the meninges, a wrapping that surrounds the brain and spinal cord. Microorganisms occasionally sneak past the blood-brain barrier and can set up shop in the cell-rich fluid that bathes the meninges. Symptoms of meningitis include fever, headache, sensitivity to light, nausea and vomiting, confusion, drowsiness, and a stiff neck. Meningitis caused by a viral infection can cause severe headaches and other symptoms but usually goes away on its own. Meningitis caused by bacteria, on the other hand, is extremely dangerous and can cause severe brain damage or death within hours to days.

Encephalitis is caused by an infection of the brain itself, although the symptoms are very similar to meningitis. Encephalitis can be caused when a virus either directly infects the brain or when a viral infection elsewhere in the body spreads to the brain. It can be caused by a variety of different viruses including herpes simplex virus, varicella-zoster virus (the cause of chicken pox and shingles), and Epstein-Barr virus (the cause of mononucleosis, or "mono").

Viruses carried by mosquitoes are the most common cause of "epidemic" encephalitis. Even though encephalitis is the most common mosquito-borne disease in the United States, this type of encephalitis is actually quite rare. Examples include encephalitis caused by West Nile virus, Eastern or Western equine virus, St. Louis virus, and La Crosse virus.