San Bernardino County health officials said the man was at the Mohave National Preserve Visitor Center last week when a brown bat landed on his neck.
The bat later tested positive for rabies.
It's believed the man who came in contact with the bat is in grave danger unless he is treated right away.
Rabies is usually fatal if not treated before symptoms occur.
Signs of rabies in animals include changes in behavior, general sickness, problems swallowing or increased drooling and aggression.
Officials have identified the mystery man as a man in his mid 50s. He left the visitor center without revealing his name but told officials he was heading to Vegas in a British accent.
Authorities are hoping to find him right away as time may be running out. If he has rabies, once symptoms of the disease occur in humans, it's usually too late.