There are more viruses in a single drop of seawater than there are people living in New York City. After replicating, viruses escape to hijack other cells, often destroying their host cells in a process called “lysis.”This isn’t always a bad thing. An ancient virus also gave rise to the human placenta, which is why our babies aren’t born from eggs. Starting in the late 1800s, scientists discovered many diseases spread by unknown, unseen pathogens.These mysterious infections were destroying tobacco plants and infecting livestock across Europe and the United States. The tobacco mosaic virus would be the first to come into actual view with the invention of the electron microscope in the 1930s.