Illustration of the anthrax toxin. Source: David Goodsell

Viruses, Bacteria and Parasites! Oh My!

Heather Kain

Posted on Feb 9th, 2017


Much confusion surrounds understanding the differences between viruses, bacteria and parasites, and their relation to disease. This blog post will help dispel this confusion and help with your understanding of infectious diseases.

Viruses are non-living - they are completely dependent on their host cell for survival and reproduction. A virus is the simplest of organisms, they are RNA or DNA wrapped-up in a protein-coat (called a capsid). That capsid may or may not be enclosed in an envelope, which is a membrane or layer that surrounds the capsid. That envelope is very important – and tricky - because the membrane is made by your own cells. That’s why our immune systems have such a hard time identifying them.

Even though the structure of viruses is simple, there are many classifications of viruses. For example, the smallpox virus (luckily eradicated) is DNA and has an envelope, while the polio virus, the smallest of all viruses, is RNA and capsid only. Having these classifications helps scientists identify viruses, which in turn helps us understand, prevent, and treat viral infections. Viruses are as common as the human cold or the flu, and can be as deadly as HIV, Ebola and Zika. However, viruses are not unique to humans – they can also affect plants and bacteria. For example, the beautiful red and yellow tulip colorings are caused by a virus, called the tulip-mosaic virus. There are bacteriophages, which are viruses that only infect bacteria and look like robotic-alien spiders.


Below: Illustration of the Zika virus. Source: David Goodsell


Unlike viruses, bacteria are living organisms. They exist as single cells; lack a nucleus and most organelles. Bacteria do not need a living host to thrive. For example, have you ever had that pinkish ring around your bathtub or toilet? That’s a bacterium named Serratia marcescens. Many types of bacteria are beneficial for us, such as the bacteria living in our digestive tract such as Escherichia coli and Peptococcus. Do you eat “probiotic” yogurt? That’s bacteria.

However, when most people think about bacteria, they think of the harmful ones that cause diseases like the plague (Yersinia pestis), TB (Mycobacterium tuberculosis) or meningitis (Streptococcus pneumonaie). Fortunately for us, we have antibiotics to treat these diseases, but antibiotic resistance is a growing problem. (So if you are prescribed antibiotics – please take the full course for the sake of all the other humans!)

Parasites are very much like people and other animals with a major exception - they survive at the expense of their host. Similar to bacteria, not all parasites are malevolent and disease causing. For instance, studies are being performed on the benefits of parasitic worms for those with asthma and/or allergies. Similarly, maggots are used to remove rotting flesh. However, there are numerous parasites that do cause significant disease, such as malaria (Plasmodium falciparum), pork tapeworm (Taenia solium), and brain-eating amoeba (Naegleria fowleri).

Studying infectious disease is absolutely fascinating, and incredibly frustrating. A major reason for this frustration is that we have an understanding and treatment for some infectious diseases and not others. For example, both the rabies and HIV viruses are similar in that they are RNA viruses with an envelope. However, rabies is 100% fatal if left untreated and those with HIV can live for a relatively long period of time. Symptoms of rabies appear quickly - but we have an effective treatment. HIV infection is asymptomatic for a long period of time. We have treatments that can prolong life, but there is only a single documented case of someone becoming “free” of the virus.

Infectious disease research is a moving target. Issues constantly arise such as drug resistance and mutation. People are often infected with more than one pathogen at a time, or have a compromised immune system making them susceptible to infections most healthy people will never acquire. The best advice I can offer a non-scientist is to be vigilant when learning about infectious disease, especially if the content is produced through mass media. Many news outlets only provide a brief overview and usually do not check data before reporting. For example, misinformation was frequently repeated during the Ebola virus outbreak. If you are going to do your own research on infectious disease issues, seek out reputable sources such as Nature or NPR. Better yet, ask an infectious disease researcher because he or she will be happy to explain.

About the Author

Heather Kain is a scientist in the Kaushansky lab and studies the pathogens that cause malaria and dengue. In her spare time she is an avid baker and hopes one day to have her own lab/bakery inside of a dormant volcano.

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