Also, it has been theorized that some proteins called prions may cause infectious diseases. Bacteria are microscopic single-celled organisms at least 1 micron long. Some bacteria species are harmless to humans, many are beneficial. But some are pathogens, including those that cause cholera, diphtheria, leprosy, plague, pneumonia, strep throat, tetanus, tuberculosis, and typhoid fever. The bacteria that are harmless and live in or on you are called resident bateria.
Viruses are tens or hundreds of times smaller than bacteria.
They are not cellular, but consist of a core of genetic material surrounded by a protective coat of protein. Viruses are able to survive and reproduce only in the living cells of a host. Once a virus invades a living cell, it directs the cell to make new virus particles. These new viruses are released into the surrounding tissues, and seek out new cells to infect. The roll call of human diseases caused by viruses includes mumps, measles, influenza, rabies, hepatitis, poliomyelitis, smallpox, AIDS, and certain types of cancer.