For a cancer patient, catching pneumonia can be deadly and, as there are around 600 different organisms that can cause pneumonia, doctors have to conduct multiple tests to identify the infection.

They are found in many organisms, including bacteria, yeasts, humans, mice, fruit flies and frogs.

"Cities are also highly connected places where trade happens, and organisms may end up on shipments in planes or boats traveling between cities."

A new Northwestern University study points to the role of gut microbes, tiny living organisms in our digestive system that help break down food and produce energy.

A multitude of factors can influence the evolution of an organism, and they are not easily predicted.