Bacteria are single-celled microorganisms. Bacteria are able to survive
in many types of environments in and on the human body. Most bacteria cause no
harm to people. For infecting organisms to survive, they must leave an existing
reservoir either within the current host or in another host and
cause infection in another place.
Bacterial infections may be marked by localized redness, inflammation,
pain or swelling.
Bacterial infections may be transmitted through direct or indirect
contact with a reservoir of infectious bacteria.
Direct contact occurs when an individual comes into contact with the
reservoir via touching infected bodily fluid; sharing beverages containing
infectious bacteria; being bitten by an insect or other animal that is carrying
the bacteria; or inhaling bacterial particles, often emitted by sneezing or
coughing. Sexual contact is a common mode of direct transmission of bacterial
Certain bacteria are able to survive outside of a host and remain
infective for extended periods of time. Indirect infection occurs when an
individual comes in contact with such an organism. These organisms may be found
anywhere, but when they exist on toys, furniture, door knobs or other personal
care products used by people, transmission is more likely to occur. Consumption
of contaminated food is another common mode of indirect transmission of
Transmission may occur through fecal-oral contact, which occurs when
sewage water is consumed or used to wash food. Such transmission often occurs
in developing countries with poor sewage or drainage systems.
The above modes of infection are examples of horizontal transmission,
which is when organisms are transmitted from person to person in the same
generation or from living person to living person. Vertical transmission occurs
when the infection is passed from mother to child during childbirth or fetal
Examples of other common bacterial infections include chlamydia, ear
infections, Helicobacter pylori, Lyme disease, methicillin-resistant
staphylococcus aureus and osteomyelitis.
Additional information about bacterial infections may be found at