Bacteria and Humans
Most bacteria are harmless to humans, and some are even beneficial. Bacteria in the human digestive tract, collectively called "gut flora," help people digest complex carbohydrates and vitamins. This would not be possible without the bacteria. But there are also disease-causing, or "pathogenic," bacteria that are always harmful, and even generally beneficial bacteria can cause harm to humans if there is too much of it in or on the body.
Conditionally Pathogenic Bacteria
Conditionally pathogenic bacteria are generally harmless. Bacteria like "Staphylococcus" and "Streptococcus" are usually present on human skin and in the nose. However, should these bacteria get into a wound or move farther into the respiratory tract, infection can result. In this case, the bacteria become harmful when they move into a different environment--from the skin to the bloodstream, or from the nose to the throat or lungs.
Intracellular bacteria, on the other hand, always cause infection. They are sometimes called "obligate intracellular parasites," because they must be in a host cell to survive and reproduce. Intracellular bacteria are bacteria that are not supposed to be in the human body, and their presence and subsequent colonization qualifies as an infection, even if the host is not bothered by any symptoms. Chlamydia and typhus are both caused by intracellular bacteria.
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Opportunistic bacteria will only cause infection in someone with a compromised immune system. These are not particularly virulent bacteria, and a healthy immune system can destroy them before any damage is done. If the person is immunocompromised because of malnutrition, disease, medical procedures or drug therapy, the immune system is unable to fight off these bacteria, resulting in an infection. The bacteria "Pseudomonas aeruginosa" is a common culprit in hospital-acquired infections, and can infect both the pulmonary system and the blood.