How Do Germs Spread

The Invisible Threat: Germs and Disease Transmission

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.

Understanding Germs

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.

Evolution and Disease Transmission

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.

The Trade-off Hypothesis

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.

Other Modes of Transmission

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.

Reducing the Harmfulness of Infectious Diseases

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.