Obvious. When you start studying the symptoms, the research process starts. But if the symptoms are not studied, then you may not have time to kill everyone, or turn people into zombies
Slopek-style. In fact, it's quite simple. The matter of routine medical examination. A lot of people just go to hospitals, where they take tests. The tests find new antibodies, a set of unknown abnormalities. Next, they take a special test, where they reveal what viruses / bacteria are present in the blood. Then they isolate our new unknown bacill and begin to study it.
Most interesting. But, if we consider that they usually discover it at the moment when most of humanity is already infected with it, then surely this virus should be widespread everywhere, and they discover it purely by accident.