Bacteria describe a class of single cell organisms. Unlike amoeba or paramecium, they do not have a distinct nucleus or other organelles. Unlike viruses, they are capable of reproducing on their own without the use of a host cell.
Bacteria exist in all environments. Most bacteria are harmless to humans, and in fact the human body hosts more far bacteria than human cells, primarily on the skin and in the digestive tract. In fact, digestive tract bacteria are vital for the proper function of the digestive system.
However, many diseases, such as tuberculosis, scarlet fever, cholera, bubonic plague, staphylococcus and most sexually transmitted diseases are transmitted by bacteria. Since the 1940s, the most common treatment of bacterial infection is the administration of antibiotics.
The body has many natural defences to bacteria. Unbroken skin will not allow bacteria into the bloodstream. The acid in the stomach will also kill most bacteria. As a result, persons with open wounds (including surgical patients) and those who use antacids on a regular basis are often susceptible to bacterial infection.Bacteria at Wikipedia