Another bonanza was the development of world trade routes, which by Roman times effectively joined the populations of Europe, Asia, and North Africa into one giant breeding ground for microbes. Today, our jet planes have made even the longest intercontinental flights briefer than the duration of any human infectious disease. Whether they take the form of spheres, rods or spirals, bacteria consist of a single cell. Unlike the cells of animals and plants, bacterial cells lack a nucleus, but they can carry out all necessary life functions. Most bacteria are parasites, although a few manufacture their own food.
Diseases can also produce toxins that damage or kill human tissue, as in food poisoning or cholera.
Sometimes bacteria in the body are helpful for a while, and then something in the body or the bacteria changes, causing destruction in the host. By far the smallest microbes, viruses can appear as spirals, 20-sided figures or even more complicated forms. They are not cells, however, and cannot carry out life functions on their own. Living inside the cells of other species, viruses use the host cells to grow and produce new viral particles. As they take over genetic material to reproduce themselves, the host cells often die.