The drive-through monkey jungle at Longleat in Wiltshire has closed after a monkey was found to have Simian B herpes, which can be fatal to humans.
Only one creature in the group, a female, was found to have the virus during routine tests of the rhesus monkey colony at the safari park.
The jungle will remain closed while the park finds out how it tested positive when all the others are negative.
The monkeys are regularly tested by the Health Protection Agency.
Keith Harris, head warden at Longleat, said: "We are well aware that Simian B herpes is dangerous for humans.
"We are being governed by the Health and Safety Executive and are taking every precaution to make sure that no-one is put in any danger.
"We expect the Monkey Jungle to be closed for the next few weeks while we and the Health Protection Agency investigate how this has happened."
The Simian B virus is part of the herpes family and is not thought to be a serious infection for monkeys - but it can be fatal if transmitted to humans.
Nigel Scott, of the Herpes Viruses Association, said: "This virus is fatal for humans and should not be confused with the everyday herpes simplex virus which commonly causes cold sores."
The monkeys were last routinely tested in October and were all found to be negative. Monkey Jungle has not been open to the public since then as it was closed for the winter.
The infected monkey is currently in quarantine.