Infections symptoms can arise from many toxins found individually or in combination, in one's environment. Historically, many forces have pushed us, with zero proof, in the direction of simply believing that the causal agent for most diseases must be a virus. Why?

A virus is invisible, can't be micrographed or isolated (unless using the term isolation as redefined by virologists), but it can kill you and must be feared. Feared so much that you must surrender your liberties to the experts who are the only ones who can save you. The virus is an incredibly effective and inexpensive weapon that the DS can use against trusting people at any moment.