How Do Infections Spread?

Long before the Dracula sneeze (aka sneeze into your elbow) came to fame, people had for a long time tried to figure out how diseases get transmitted. This is not just scientific curiosity: it plays a major role in what will be effective in protecting people, especially as we started understanding better the role of microbes.

In order to infect an organism, microbes need to find a way to get into it. From the tiniest scratch on our skin to our mouth and nose, through which we breathe and ingest food and drink daily, there are many “doors” into our bodies. But microbes evolved to make use of specific doors – and sometimes using another organism to reach ours, as we saw with mosquitoes and malaria.

Transmission Through the Air

Very small microbes, such as viruses, can be transported in the air or in droplets and breathed in. Such microbes are called air-borne. This is the case of measles, which is extremely contagious. Not all air-borne diseases are very contagious or very severe. Tuberculosis, a very deadly disease in Belgium throughout the 19th and part of the 20th century, often will not affect healthy people living in healthy environments. Other famous air-borne diseases are COVID-19 as well as the common cold.

Food and Drink

Food and drink are very effective vehicles for microbes, as we all probably have experiences with the gastric flu. Cholera is one such disease: when sewage is not well separated from the water that people consume, the bacterium that causes it can easily spread. Like others, such as typhoid fever, salmonellosis, and many others, water that is ingested is the main culprit, but food preparation in unhygienic conditions can also permit the microbe to reach another person.

Parasitic Transmission

Not all water-borne diseases are ingested, however: schistosomiasis is a parasitic disease caused by a tiny worm that enters the human body when people get in contact with water bodies in which the parasite – and its intermediate host, a snail, are found. In many regions of the world where this parasite and its snail host are found, water distribution is scant and people use lakes and rivers for washing and doing laundry.

Fomites

Fomites – that is any inanimate object on which microbes can survive for a while, can be responsible for disease transmission. This is notably the case for influenza. Think of using a door handle then rubbing your eyes or nose for example.

There are many other routes for infection, such as transplacental, sexual, blood-borne. What it shows us is that microbes are incredibly diverse and, like other animals, they survive by adapting to their environment through forces driven by evolution. Just like foxes have adapted to cities, microbes adapt to whatever habitat they come across. But this is not just natural history: understanding the intimate life of microbes can help us avoid transmission. As these few examples show, means to avoid transmission relate to how people behave, for example washing your hands after using the toilet and before preparing food, but also how life is organized and what infrastructures exist to permit the availability of clean water. We may forget it, but our cities bear the history of public health, as rivers got covered, sewers built, mobility organized, housing controlled and regulated.

Explore more articles What is an epidemic – How Do Infections Spread?– Mosquito-borne Diseases – Herd Immunity – Koch’s Postulates