With health and infectious disease concerns sweeping the world over the past few years, it might be concerning to learn that causes of disease can be anywhere and everywhere in our lives. However, it’s also important to know that many similar harmless microorganisms are just as, if not more, abundant than harmful microorganisms, and while it might take many years to learn all about every single one, here is just a quick breakdown on the main categories of these agents of disease.
Bacteria are single-celled organisms that are found all throughout our planet, from the coldest of poles to the deepest of hydrothermal vents. Some have even been found to originate from outer space! However, one of the most widely known habitats for bacteria is the human body. And while it may be alarming to learn that these tiny microorganisms exist in areas like your mouth, gut, and all over your skin, you should be relieved to know that the majority of these bacteria are not harmful in the least. In fact, some might even provide benefits to you, like helping you digest certain foods or produce vitamins for your body to use. Some very prominent bacteria include Yersinia pestis, the causative agent of the Black Plague, and Clostridium botulinum, the producers of the deadly botulinum toxin, which is also used for medical and cosmetic procedures.
Viruses, on the other hand, are not cells at all. They’re simply nucleic acids, like DNA or RNA, packaged in protein, and sometimes bits of their host cells that they’ve taken after infecting said cell. Unlike bacteria, viruses can’t reproduce outside of the cells of other beings, and they don’t provide the benefits of bacteria, like vitamin production or food digestion. In fact, viruses have been shown to be the causative agents of some of the world’s worst diseases, like AIDS, Ebola, and COVID19. Like bacteria, they exist in great quantities on and in the human body, although thankfully, the majority of them are incapable of infecting your cells.
Finally, parasites are a large group of organisms that can be single-celled, like protozoa, or multicellular like ringworms. The commonality between these organisms is that they typically live, reproduce, and thrive at the cost of their hosts, as well as usually residing in or on them. Parasitic protozoa, such as members of the Cryptosporidium genus cause diarrheal diseases and even death in very young or immunocompromised people. Another, much larger parasite, the tapeworm, is typically ingested and absorbs nutrients from its hosts, growing up to 15 meters on occasion!
While it might be daunting to know that the world is full of disease-causing organisms, and that you’re practically crawling with microbes, it may be reassuring to be able to differentiate between them and know that many of these tiny things are harmless. But as usual, always remember to wash your hands and cook your food thoroughly to ensure limited contact with harmful, disease-causing microbes!

References
1.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC321258/
2.https://www.frontiersin.org/article/10.3389/fmicb.2013.00124
3.https://doi.org/10.1186/s40168-019-0666-x
4.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7149683/
5.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4878258/
6.https://www.nature.com/articles/s41564-019-0539-x
7.http://www.ncbi.nlm.nih.gov/books/NBK537154/
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