The one of the following diseases which is caused by a virus is?

The correct answer is encephalitis because it is primarily caused by various viruses. Encephalitis is an inflammation of the brain that can result from viral infections, such as those caused by herpes simplex virus, West Nile virus, and other arboviruses. This condition often presents with symptoms like headache, fever, confusion, and in severe cases, seizures or loss of consciousness.

On the other hand, malaria is a disease caused by parasites of the Plasmodium species, which are transmitted through mosquito bites. Q fever is caused by the bacterium Coxiella burnetii, typically associated with livestock. Tetanus is caused by the toxin of the bacterium Clostridium tetani, which is found in soil, dust, and animal feces. Each of these other options stems from different biological origins, primarily bacteria or parasites, making encephalitis the clear choice related to viral causation.