Esther Lederberg (1922-2006) studied bacteria and viruses, helping her work by inventing a technique called replica plating, which made it easy to study certain bacterial colonies across a set of Petri dishes.
The technique contributed to a Nobel Prize for her husband.
From this work, she confirmed that bacteria mutate randomly, including acquiring resistance to particular antibiotics before ever having been exposed to that particular chemical.
She also discovered a type of virus called a lambda phage, which lies low in a cell until the cell is going to die from other causes. It's now used as a model for human viruses like herpes and tumor viruses.