Dr. Greene's Answer
Viruses are tiny geometric structures that can only reproduce inside a living cell. They range in size from 20 to 250 nanometers (one nanometer is one billionth of a meter). Outside of a living cell, a virus is dormant, but once inside, it takes over the resources of the host cell and begins the production of more virus particles. Viruses are more similar to mechanized bits of information, or robots, than to animal life.
Bacteria are one-celled living organisms. The average bacterium is 1,000 nanometers long. (If a bacterium were my size, a typical virus particle would look like a tiny mouse-robot. If an average virus were my size, a bacterium would be the size of a dinosaur over ten stories tall. Bacteria and viruses are not peers!) All bacteria are surrounded by a cell wall. They can reproduce independently, and inhabit virtually every environment on earth, including soil, water, hot springs, ice packs, and the bodies of plants and animals.
Most bacteria are harmless to humans. In fact, many are quite beneficial. The bacteria in the environment are essential for the breakdown of organic waste and the recycling of elements in the biosphere. Bacteria that normally live in humans can prevent infections and produce substances we need, such as vitamin K. It is estimated that 1000 to 2000 different species of bacteria live in the human gut and skin. It is also impressive to know that there are at least ten times as many bacteria as human cells living in the human body. Bacteria in the stomachs of cows and sheep are what enable them to digest grass. Bacteria are also essential to the production of yogurt, cheese, and pickles. They are now also commonly included as probiotic dietary supplements. Some bacteria cause infections in humans. In fact, they are a devastating cause of human disease.