Bacterial Infection Definition and Common Examples

Bacterial infections are varied and exceedingly common

Streptococcus pneumoniae artwork
Streptococcus pneumoniae artwork. Getty Images/HIPERSYNTEZA/SCIENCE PHOTO LIBRARY

A bacterial infection is an illness due to pathogenic bacteria that gain entrance to the body through various means, including the digestive system, the respiratory system,​ and the skin. Once inside the body, the bacteria reproduce, which may lead to various symptoms and illnesses. Bacterial infections are often treated with antibiotics.

In Our Own Words:

A bacterial infection is an illness caused by bacteria.

Bacteria are single-celled organisms. Most types of bacteria do not cause harm to humans, but about 1 percent can lead to illness and make someone sick. Bacteria get into the body in different ways. In some cases, bacteria gets in through a break in the skin or through the nose. Bacteria can also enter the body through eating contaminated food.

Once inside the body, the bacteria multiply, and some give off specific toxins, which can add to the illness. As opposed to viral infections, bacterial infections are usually treated with antibiotics.

Some Common Examples of Bacterial Infections

Pneumonia. Pneumonia is usually caused by bacterial infection of the lungs (technically the lung parenchyma or portion of the lung involved in the exchange of blood gases). Pneumonia is a significant cause of disease and death throughout the world. Unfortunately, pneumonia is often misdiagnosed or mistreated.

Broadly, this bacterial infection can be characterized as healthcare-associated pneumonia (HCAP) or community-acquired pneumonia (CAP). In the past, HCAP was most often associated with particularly severe cases of pneumonia caused by multidrug resistant pathogens or bacteria. However, because of the increased use and misuse of antibiotics as well as earlier discharge of patients from hospital setting, we're seeing more resistant strains of pneumonia among the general population.

Urinary tract infections. A urinary tract infection (UTI) is caused by the introduction of bacteria from your private parts into the urinary system and is a common cause of bacterial infection worldwide. Due to anatomical differences between women and men--women have a shorter urethra--women are far more likely to experience UTIs than are men. Although more routine antibiotic therapy often effectively treats UTIs, much like pneumonia and other types of bacterial infections, multidrug resistance is also popping up as a more recalcitrant cause of UTI.  

Cellulitis. Cellulitis is mostly caused by a bacterial infection of the skin or soft tissue underlying the skin. Cellulitis can cause exquisitely painful boils (abscesses). Cellulitis results in pain and swelling of the skin and is very common--about 2 percent of all hospital admissions are due to cellulitis. It can take several days for cellulitis to resolve after treatment with antibiotics.

Diarrhea. You're probably well acquainted with diarrhea.

In the United States, nearly 100 million people experience diarrhea every year. Greater than 90 percent of diarrhea cases are attributable to infection, such as bacteria and viruses. Bacteria that causes diarrhea is typically introduced into the digestive system by ingestion of food contaminated with bacteria.


University of Colorado Hospital. Care and Treatment for Bacterial Infections.” Accessed February 2014.

Tortora G., Derrick B. “Principals of Anatomy and Physiology.” Wiley 2011. Accessed February 2014.

University of Rochester Medical Center. “Is it a Virus or Bacterium? Know the Difference.”. Accessed February 2014.

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