We conducted a literature search in the Medline database using the PubMed website. The incidence of presumably infectious encephalitis is estimated at 1.5–7 cases per 100,000 inhabitants per year, excluding epidemics. Infectious encephalitis and immune-mediated encephalitis share similar clinical signs and symptoms. The latter accounts for a significant proportion of cases of presumably infectious encephalitis without an established etiological diagnosis, as demonstrated by a prospective cohort study in which 21% of cases were due to an immune cause. Several infectious agents are frequently reported across all studies: Herpes simplex virus (HSV) is the most common pathogen in 65% of studies, followed by Varicella-zoster virus (VZV) in several studies. Enteroviruses are also reported; they were the most common viruses in two studies and the second or third most common in five other studies. There are significant regional differences, particularly regarding vector-borne transmission: Asia and the Japanese encephalitis virus; Eastern and Northern Europe/Eastern Russia and the tick-borne encephalitis virus; and North America and Flavivirus or Alphavirus. Bacteria can also be implicated: Mycobacterium tuberculosis and Listeria monocytogenes are the most common, after HSV and VZV, in a French prospective study. The epidemiology of encephalitis is constantly evolving. Epidemiological data may indicate the emergence and/or spread of new causative agents. The spread and emergence of causative agents are driven by environmental, social, and economic changes, but prevention programs (vaccination, vector control) help reduce the incidence of other infectious diseases and associated encephalitis (e.g., measles).