Anyone can develop encephalitis. Factors that may increase the risk include: Age. Some types of encephalitis are more common or more severe in certain age groups. In general, young children and older adults are at greater risk of most types of viral encephalitis. Weakened immune system. People who have HIV/AIDS, take immune-suppressing drugs or have another condition causing a weakened immune system are at increased risk of encephalitis. Geographical regions. Mosquito- or tick-borne viruses are common in particular geographical regions. Season of the year. Mosquito- and tick-borne diseases tend to be more common in summer in many areas of the United States.