B-virus (Macacine alphaherpesvirus 1; McHV-1; formerly Macacine herpesvirus 1, Cercopithecine herpesvirus 1, CHV-1), Herpesvirus simiae, or Herpes virus B is the Simplexvirus infecting macaque monkeys. B virus is very similar to HSV-1, and as such, this neurotropic virus is not found in the blood.

In the natural host, the virus exhibits pathogenesis similar to that of cold sores in humans. Conversely, when humans are zoonotically infected with B virus, patients can present with a severe encephalitis, resulting in permanent neurological dysfunction or death. Severity of the disease increases for untreated patients, with a case fatality rate of approximately 80%. Early diagnosis and subsequent treatment are crucial to human survival of the infection.

B virus is the only identified old-world-monkey herpesvirus that displays severe pathogenicity in humans. There have been a number of accidental infections and fatalities of researchers working with rhesus monkeys (Rhesus macaque). Personal protective equipment is necessary when working with macaques, especially with animals that have tested positive for the virus. Bites, scratches, and exposures to mucous membranes, including the eye, have led to infection when not cleaned immediately.

B virus infection is extremely rare, but it can lead to severe brain damage or death if it is not treated immediately. People typically get infected with B virus if they are bitten or scratched by an infected macaque monkey, or have contact with the monkey's eyes, nose, or mouth. Only one case has been documented of an infected person spreading B virus to another person. As of 2020, there have been 50 documented cases of human B virus infection since the identification of the virus in 1932, 21 of which led to death. At least 20 of the patients developed some degree of encephalitis.