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Understand  –  Bacteria

Bacteria and infections

Throughout history, bacterial infections have played a central role in the lives and deaths of humans.

Bacteria cause many common infections such as pneumonia, wound infections, bloodstream infections (sepsis) and sexually transmitted diseases like gonorrhea, and have also been responsible for several major disease epidemics. One example from the 1340’s is the plague, also known as the “Black death”, that spread across Asia and Europe along the trading routes, killing millions of people. The disease is now known to be caused by the bacterium Yersinia pestis and is treatable by antibiotics.

When do bacteria cause disease?

Bacteria can be strictly pathogenic, which means that they will cause disease if they manage to overwhelm the human immune system. Other bacteria only cause disease given the right circumstances, these are so-called opportunistic pathogens. Opportunistic pathogens normally do not cause infections in healthy humans but when the immune system is compromised or suppressed by for example cancer chemotherapy, other diseases (like HIV/AIDS) or malnutrition, the risk of infection increases. These infections often originate from the individual’s own bacterial flora such as that on the skin or in the gut. Many bacterial pathogens can spread and infect via water and food, including Salmonella, Campylobacter and E. coli. Sometimes bacteria are transmitted directly or indirectly from animals to humans and cause disease. Such infections are called zoonotic infections. Other bacteria like Neisseria gonorrhea and Chlamydia trachomatis spread via sexual contacts.

Managing bacterial infections

The introduction of antibiotics to treat bacterial infections in combination with improved hygiene and sanitation, use of preventive vaccinations as well as increased knowledge about bacteria have greatly reduced deaths from bacterial diseases. However, antibiotic resistance among bacteria is now threatening to again leave us without effective treatments for many common bacterial infections. Resistant bacteria are now widespread in most parts of the world and more and more people die from bacterial infections because the antibiotics have stopped working. For information about antibiotic resistance, see Antibiotic resistance, and the sub-sections that follows.

Identifying the disease

Diagnostics are used to determine what disease a patient is afflicted by, and in the case of infectious diseases what is causing the disease. This information is then used to determine what the appropriate therapy is: for example a specific antibiotic, other medicines or bed rest. In the absence of a proper diagnosis many patients may not get the correct antibiotic treatment when they need it, but another common problem is that patients who do not need antibiotics are given one just in case it is a bacterial infection.

Below are a set of resources about bacterial diseases and their spread.

Selected Resources

Resource Description
What are infectious diseases? Article. Facts about infectious diseases, what they are, and an introduction to the different groups of microorganisms that cause them (bacteria, viruses, fungi and parasites).
Bacteria in medicine (Encyclopaedia britannica) Article about bacterial diseases and their impact. Also provides links to additional pages about specific bacteria.
Understanding Microbes in Sickness and in Health Booklet. Learn about microbes and their role in health and disease in this book written for the general public.
10 facts about zoonotic neglected tropical diseases Facts about the impact and spread of neglected tropical diseases like rabies and porcine tapeworm infection.
Zoonotic Diseases Fact sheet that explains more about common zoonotic infections – infections shared between animals and humans – and how they can spread (focus on the US). developed by the CDC.
The Black Death: The Greatest Catastrophe Ever Article about “The Black Death”, the plague epidemic in the 1300s.

More from "Bacteria"

Bacteria - Bacteria in medicine [Internet]. Encyclopedia Britannica. [cited 2019 Aug 15]. Available from:
Centers for Disease Control and Prevention - CDC. Zoonotic Diseases [Internet]. [cited 2018 Dec 7]. Available from:
World Health Organization - WHO. 10 facts about zoonotic neglected tropical diseases [Internet]. WHO. [cited 2018 Dec 7]. Available from:
National Institute of Allergy and Infectious Diseases - NIAID, U.S. Department of Health and Human Services. Understanding Microbes in Sickness and in Health - microbesbook.pdf [Internet]. 2009 [cited 2015 Aug 19]. Available from:
yourgenome. What are infectious diseases? [Internet]. 2014 [cited 2015 Feb 21]. Available from:
Benedictow O. The Black Death: The Greatest Catastrophe Ever. History Today [Internet]. 2005 Mar;55(3). Available from: