Infectious agents that affect the nervous system can cause their pathology by direct means, such as in meningitis or encephalitis, or indirectly (parainfectious), either from immunologic reactions (Guillain-Barre syndrome, acute disseminated encephalomyelitis) or toxin-mediated reactions (tetanus, botulism). The epidemiology of bacterial meningitis has changed significantly over the past 10 years with the near eradication ofHaemophilus influenzaein the Western world and the emergence of penicillin-resistantStreptococcus pneumoniae.Herpes simplex remains the most common sporadic encephalitis in the United States, with the arboviruses being the most common overall. Human herpes virus 6 is an emerging cause of encephalitis in the immunocompetent and immunodeficient. Rabies remains a rare cause of encephalitis in the United States, is still common worldwide, and has no effective treatment Subacute sclerosing panencephalitis is also rare since the introduction of the measles vaccine but is almost always fatal. A promising treatment regimen for subacute sclerosing panencephalitis is discussed in this review. Two recent studies on the treatment of Guillain-Barre syndrome demonstrate that plasmapheresis and intravenous immunoglobulin are equally effective and that no more than four plasma exchanges are necessary. Acute disseminated encephalomyelitis remains rare and is most common in young adults. Tetanus is still prevalent in developing countries where facilities for critical care are scarce, and for this reason the mortality rate remains very high. Botulism toxin is one of the most potent toxins known and is used both therapeutically and as an agent in biological warfare.