For much of human history, we didn’t understand what caused and spread sickness. Perfectly healthy people would suddenly take ill and plagues would destroy whole cities. But people had no concept of why such terrible things happened. They blamed disease on unseen forces, angry spirits, and vengeful gods.
It’s not surprising that we once blamed invisible powers for causing disease, because the actual sources for many illnesses are practically invisible. However, they are organic, living organisms -- microbes, including bacteria and viruses. Medical microbiologists and bacteriologists study these microorganisms.