Which Diseases Spread the Fastest?

When the COVID pandemic hit, many people turned to the eerily prescient film Contagion (2011) for answers—or at least for catharsis.

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Suddenly, its hypothetical plot felt all too real. Applauded for its scientific accuracy, the film offered more than suspense—it offered lessons.

One scene in particular stands out. Kate Winslet's character delivers a concise lesson on the infectious power of various pathogens—explaining how they can be spread from our hands to the many objects we encounter each day—“doorknobs, water fountains, elevator buttons and each other.” These everyday objects, known as fomites, can become silent vehicles for infection.

She also considered how each infection is given a value called R0 (or R-nought) based on how many other people are likely to become infected from another. So, for an R0 of two, each infected patient will spread the disease to two others. Who will collectively then give it to four more. And so a breakout unfolds.

The R0 measure indicates how an infection will spread in a population. If it's greater than one (as seen above), the outcome is disease spread. An R0 of one means the level of people being infected will remain stable, and if it's less than one, the disease will often die out with time.

Circulating infections spread through a variety of routes and differ widely in how contagious they are. Some are transmitted via droplets or aerosols—such as those released through coughing or sneezing—while others spread through blood, insects (like ticks and mosquitoes), or contaminated food and water.