The sun is shining, the birds are singing, and it seems like the start of another beautiful day. You’re strolling through the park when suddenly, a stranger sneezes nearby. You can feel the droplets of moisture land on your skin, but what you can’t feel are the thousands, or even millions, of microscopic germs that have covertly traveled through the air and onto your clothing, hands, and face. As unpleasant as this scenario sounds, it’s actually very common for our bodies to be exposed to disease-causing germs, and most of the time, it’s not nearly as obvious. Germs are found on almost every surface we come into contact with.
When we talk about germs, we’re actually referring to many different kinds of microscopic organisms, including bacteria, fungi, protozoa, and viruses. All germs have the ability to interact with our bodies and change how we feel and function. Scientists who study infectious diseases have wondered for decades why some of these germs are relatively harmless, while others cause devastating effects and can sometimes be fatal. We still haven’t solved the entire puzzle, but we do know that the harmfulness, or virulence, of a germ is a result of evolution.
How can it be that the same evolutionary process can produce germs that cause very different levels of harm? The answer starts to become clear if we think about a germ’s mode of transmission, which is the strategy it uses to get from one host to the next. A common mode of transmission occurs through the air, like the sneeze you just witnessed, and one germ that uses this method is the rhinovirus, which replicates in our upper airways, and is responsible for up to half of all common colds.
Imagine that after the sneeze, one of three hypothetical varieties of rhinovirus, let’s call them “too much,” “too little,” and “just right,” has been lucky enough to land on you. These viruses are hardwired to replicate, but because of genetic differences, they will do so at different rates. This describes what scientists call the trade-off hypothesis. First developed in the early 1980s, it predicts that germs will evolve to maximize their overall success by achieving a balance between replicating within a host, which causes virulence, and transmission to a new host.
It would be great if the story ended there, but germs use many other modes of transmission. For example, the malaria parasite, plasmodium, is transmitted by mosquitoes. Unlike the rhinovirus, it doesn’t need us to be up and about, and may even benefit from harming us since a sick and immobile person is easier for mosquitoes to bite. We would expect germs that depend less on host mobility, like those transmitted by insects, water, or food, to cause more severe symptoms.
So, what can we do to reduce the harmfulness of infectious diseases? Evolutionary biologist Dr. Paul Ewald has suggested that we can actually direct their evolution through simple disease-control methods. By mosquito-proofing houses, establishing clean water systems, or staying home when we get a cold, we can obstruct the transmission strategies of harmful germs while creating a greater dependence on host mobility. So, while traditional methods of trying to eradicate germs may only breed stronger ones in the long run, this innovative approach of encouraging them to evolve milder forms could be a win-win situation. Well, for the most part.
Using a harmless powder that glows under UV light, simulate how germs can spread from person to person. Observe how quickly and widely the “germs” spread in a controlled environment. This will help you understand how easily germs can be transmitted.
Choose a type of germ (bacteria, fungi, protozoa, or virus) and conduct a research project on it. Present your findings to the class, focusing on its mode of transmission and the diseases it can cause.
Engage in a group discussion about the role of evolution in disease transmission. Discuss the trade-off hypothesis and how it applies to the germs you researched in Activity 2.
Brainstorm and discuss different disease control methods that can be used to reduce the harmfulness of infectious diseases. Consider both traditional methods and innovative approaches like those suggested by Dr. Paul Ewald.
Create a public health campaign to educate others about germs and disease transmission. This could include creating posters, brochures, or a short video. Remember to include information about how to prevent the spread of germs and the importance of disease control methods.
Germs – Microorganisms, especially bacteria, that can cause disease. – Be sure to wash your hands regularly to kill any germs that you may have picked up.
Disease transmission – The transfer or spread of a disease from one person, animal, or object to another. – The most common modes of disease transmission are through direct contact or through the air.
Bacteria – Single-celled microorganisms that can exist as independent organisms or as parasites and can cause various infections and diseases. – Streptococcus and Escherichia coli are examples of bacteria that can cause illnesses.
Fungi – A group of organisms that includes yeasts, molds, and mushrooms, which can cause infections in humans and other animals. – Athlete’s foot is a common fungal infection that affects the skin between the toes.
Protozoa – Single-celled microorganisms that can cause diseases such as malaria and dysentery. – The protozoan parasite Plasmodium is responsible for transmitting malaria to humans.
Viruses – Submicroscopic infectious agents that replicate themselves inside the cells of living hosts and can cause diseases such as the flu and COVID-19. – The common cold is caused by a viral infection.
Evolution – The process of gradual change in a species over generations, resulting in the development of new characteristics and species. – Evolution is driven by natural selection and allows organisms to adapt to their changing environments.
Mode of transmission – The specific way in which an infectious agent is passed from one person, animal, or object to another. – The mode of transmission for COVID-19 is primarily through respiratory droplets when an infected person coughs or sneezes.
Trade-off hypothesis – A theory suggesting that pathogens (disease-causing organisms) evolve to balance their infectiousness with their harm to the host, as excessively harmful pathogens may lead to host death before transmission. – The trade-off hypothesis explains why some infectious diseases cause mild symptoms, while others can be severe or fatal.
Malaria parasite – A type of protozoan parasite of the genus Plasmodium that causes malaria in humans. – The malaria parasite is transmitted to humans through the bite of infected mosquitoes.
Reducing harmfulness – The process of minimizing the severity and impact of a disease or infection on an individual or population. – Vaccinations play a crucial role in reducing the harmfulness of certain infectious diseases by stimulating the body’s immune response.
Infectious diseases – Diseases caused by pathogenic microorganisms or parasites that can be transmitted from one individual to another. – Examples of infectious diseases include influenza, tuberculosis, and HIV/AIDS.
