Bacteria are one-celled organisms that can be seen only with a microscope. Not all bacteria are harmful, and some bacteria that live in your body are helpful. However, many bacteria produce toxins that damage cells and make you ill. Other bacteria can directly invade and damage tissues.
Viruses are much smaller than cells. They are basically capsules that contain genetic material. To reproduce, viruses invade cells in your body, hijacking the machinery that makes cells work. Often, host cells are eventually destroyed during this process.