Anyone can develop encephalitis (inflammation of the brain), although it is highly unlikely. Age is a major risk factor for encephalitis, with the very young and the elderly being at particular risk. This is because these populations typically have developing or weaker immune systems than other people, leaving them more vulnerable to infection.

Other factors that increase the risk of encephalitis include:

• Immune system problems. People with relatively weak immune systems are at greater risk of contracting encephalitis than otherwise healthy individuals. These include people with HIV or AIDS, cancer and other conditions, as well as those taking medications to suppress the immune system (e.g., after an organ transplant).
  
  
• Exposure to metals. Poisoning by certain metals, such as lead (lead encephalitis), can cause encephalitis in some individuals.

In addition, a number of risk factors apply only to the forms of encephalitis caused by contact with mosquitoes and ticks (arthropod-borne encephalitis). These include:

• Time of year. Because birds and mosquitoes tend to mate in the warmer summer months, this time of the year is also when most cases of encephalitis are reported.
  
  
• Geography. Certain locations are more suitable for mosquitoes and ticks than others. Mosquitoes often lay their eggs in areas with standing water, whereas ticks usually live in areas with dense vegetation. Children who visit or live in such areas are at increased risk of contracting encephalitis.
  
  
• Outdoor activity. Children who spend a proportionately large amount of time outdoors are at greater risk for contracting arthropod-borne encephalitis, particularly during an outbreak.

Cases of arthropod-borne encephalitis tend to occur in clusters within certain regions or time periods, and therefore, are better-known forms of the disease. However, they are less common than the other potential causes of encephalitis, including:

• Herpes simplex virus (HSV). This infection can lead to herpes simplex encephalitis (HSE). Though uncommon, HSE is the leading cause of fatal encephalitis cases in the United States. Up to half of newborns whose brains are infected with this virus will die from it, according to the American Academy of Physicians. Two types of HSV infections can cause HSE:
  
  
• • HSV type 1 (HSV-1) is generally recognized as the virus that causes the common cold sores or fever blisters around the mouth area. Although incredibly rare (affecting approximately two out of every 1 million people each year), HSV-1 is responsible for nearly all cases of HSE in older children and adults. However, studies have shown that people with HSV-1 who have cold sores are no more likely to contract encephalitis than those with HSV-1 who do not have cold sores.
    
    In addition, some people may have a genetic tendency for HSE caused by HSV-1, according to a recent study. Researchers from the Necker Medical School in Paris, France have identified a single gene (UNC93B1) that predisposes individuals to encephalitis caused by HSV-1. People who inherit two mutated copies of the UNC93B1 gene appear to be at higher risk of contracting encephalitis from an HSV-1 infection. Results from the study also revealed that a significant portion of patients with HSE caused by HSV-1 had parents who were blood relatives (e.g., first or second cousins) and were therefore at higher than normal risk of inheriting two copies of the faulty gene. 
    
    
  • HSV type 2 (HSV-2) is more commonly known as the virus that causes genital herpes. Newborns can contract encephalitis at the time of delivery if the mother has active HSV-2.
    
    
• Childhood infections such as German measles (rubella), measles (rubeola) and mumps. In rare instances, children can develop encephalitis after recovering from these conditions. When encephalitis does occur, it is usually in reaction to antibodies that have developed in the immune system. Encephalitis infection in children under these circumstances tends to be moderate.
  
  
• Cytomegalovirus (CMV) infection. CMV is a member of the herpes virus group and is transmitted through contact with infected body fluids (e.g., saliva, urine, semen, breast milk). The Centers for Disease Control and Prevention (CDC) estimates that between 50 and 85 percent of adults in the United States contract CMV by the age of 40, although most people never develop symptoms. In rare cases, encephalitis can be caused by CMV.
  
  
• Varicella-zoster virus. Responsible for chickenpox and shingles, this virus can cause what tends to usually be mild encephalitis in children and adults.
  
  
• Epstein-Barr virus. Known to cause mononucleosis, Epstein-Barr virus can also cause fatal encephalitis. However, the majority of encephalitis cases that result from Epstein-Barr virus tend to be comparatively less severe than other cases of encephalitis.
  
  
• HIV (human immunodeficiency virus). HIV, the virus that causes acquired immune deficiency syndrome (AIDS) can cause encephalitis in patients, principally in the early stages of illness.
  
  
• Rabies. Although incidences of rabies are extremely rare, raccoons are the most common carriers of the disease in the United States. Occasionally, rabies can cause encephalitis in humans.  
  
  
• Bacterial infections. In rare cases, infections such as bacterial meningitis can cause encephalitis.
  
  
• Toxoplasmosis. Caused by the toxoplasma gondii parasite. People can get toxoplasmosis through infected food or water or from coming in contact with infected cat feces. In rare case, toxoplasmosis can cause encephalitis.
  
  
• Roundworm infection. This usually occurs as a result of exposure to raccoon feces.