There are a number of viruses which change the behavior of the infected host, to varying degrees. A "zombie state" is probably best approximated by rabies. Symptoms include (among others) hallucinations and increased aggression, leading to the infected individual biting others. There is also evidence that the influenza virus ("the flu") might increase sociability. A few other organisms are known to alter cognitive function by destroying brain tissue, as can prions. However, full "mind control", wherein the host is driven to complete a specific set of actions, seems to occur only in animals other than humans.

Rabies causes some zombie-like behavior, and there are other diseases that do similar things in other types of animals (e.g. the Cordyceps fungus afflicts insects ). Nothing allows somebody else to control you, though.

Wow, that is an exciting question!

Although I don’t know of any virus that turns people into zombies, I do know of a fungus which turns ants into zombies. There is a special fungi of the genus Ophiocordyceps, which infects certain species of ants and controls their actions, effectively turning them into zombies. This is called “behavioral manipulation.” In the case of these ants, the fungus takes over their nervous system and makes the ants climb up onto a twig or leaf. The fungus then kills the ant and grows a spore out of the ant’s body. The spore then spreads the fungus to other ants. Although this fungus does not infect humans (that we know of!), there are diseases I know which can alter behavior of humans, which is pretty close to being mind controlled! The most prominent example I can think of is rabies.

Rabies is a viral infection which affects the brain. It can cause insomnia, confusion, aggressive or abnormal behavior, hallucinations, among other symptoms. It may also cause hydrophobia. Scientists have noted that many of these symptoms aid the spread of the virus. For example, aggressive behavior makes infected individuals more likely to come into close contact with other animals which the virus can infect. Similarly, since the virus makes it so the victim cannot drink, and the thought of drinking or swallowing may cause spasms of muscles in the throat, excess saliva cannot be swallowed and again the virus has a higher chance of spreading. In this sense, the virus is very much like a zombie infection in that it turns its host into a vehicle to infect further victims.

Another example of this is the parasite Toxoplasma gondii. The disease caused by infection with this parasite is called toxoplasmosis. The parasite is known to infect rodents and make them lose their fear of cats, which is advantageous to the parasite because it breeds in the guts of cats. Although this parasite is known to infect human brains, scientists do not agree on whether or not it has any effect on the behavior of human hosts.

Other diseases which affect the brain and nervous system, and thereby the behavior of victims that I was able to find include prion diseases and subacute sclerosing panencephalitis. The study of diseases is called pathology, and there are all kinds of diseases which we don’t know about yet. Maybe one day you will discover a new one!