These Are The Fastest-Spreading Diseases, According to Science

When the COVID pandemic arrived, countless people revisited the unsettlingly foresighted film Contagion (2011) in search of insight - or, at the very least, emotional release. All at once, what had been a fictional scenario felt uncomfortably familiar.

Widely praised for getting the science right, the film delivered more than tension: it also communicated practical takeaways.

A standout moment features Kate Winslet’s character offering a sharp explanation of just how readily different pathogens can pass between people - including via the objects we touch all day long - "door knobs, water fountains, elevator buttons and each other". These ordinary surfaces, known as fomites, can act as unnoticed carriers of infection.

In the same exchange, she notes that every infection can be assigned a value called R0 (or R-nought), based on how many additional people one infected person is likely to infect.

With an R0 of two, for instance, each infected patient would pass the disease to two others, who would then collectively infect four more - and that is how an outbreak gathers momentum.

R0 is essentially a guide to how an infection is expected to move through a population. When it exceeds one (as in the example above), infections tend to increase; at one, case numbers typically hold steady; and below one, an outbreak will often fade over time.

Infectious diseases circulate through many different routes, and they vary hugely in contagiousness. Some spread through droplets or aerosols - such as those produced by coughing or sneezing - whereas others are transmitted via blood, insects (including ticks and mosquitoes), or contaminated food and water.

Stepping back, a key part of protecting ourselves from infectious disease is understanding the ways infections spread. As we will see, that understanding is also central to protecting other people - not only ourselves.

Below is an overview of some of the most and least infectious diseases worldwide.

The top spot for contagiousness goes to measles.

In recent years, measles has returned in many parts of the world, including high-income countries such as the UK and US. Although multiple forces are involved, the main driver has been falling childhood vaccination rates. That decline has been fuelled by disruptions including the COVID pandemic and global conflict, alongside the spread of misinformation about vaccine safety.

For measles, the R0 lies between 12 and 18. If you work it through, just two rounds of transmission from a single initial case could result in 342 people becoming infected. That is an astonishing figure originating from one patient - although, fortunately, vaccination reduces real-world spread by decreasing the number of people who are susceptible.

Measles is exceptionally virulent, transmitted through tiny airborne particles released when an infected person coughs or sneezes. Direct contact is not even necessary. It is so infectious that an unvaccinated person can contract the virus simply by walking into a room where someone with measles was present two hours earlier.

People may also transmit the virus before they develop symptoms - and before they have any reason to isolate.

Other infections with high R0 values include pertussis, or whooping cough (12 to 17), chickenpox (ten to 12), and COVID, which differs by subtype but generally sits between eight and 12. Many people recover fully from these illnesses, yet they can still cause severe complications, including pneumonia, seizures, meningitis, blindness and, in some instances, death.

Low spread, high stakes: R0 and tuberculosis (TB)

At the opposite end of the scale, a lower infectivity does not make a disease harmless.

Consider tuberculosis (TB), which can have an R0 from less than one up to four. Where a community falls within that range depends on local conditions, such as housing and crowding, as well as the standard of healthcare that is available.

TB is caused by the bacterium Mycobacterium tuberculosis. It is also airborne, but it typically spreads at a slower pace and usually requires extended close contact with someone who has active disease. Outbreaks are therefore more likely among people sharing living environments - for example families and households, as well as those in shelters or prisons.

The major threat from TB is how challenging it can be to treat. Once the infection takes hold, it demands a regimen of four antibiotics for at least six months. Common antibiotics such as penicillin do not work, and TB can also disseminate beyond the lungs to other organs and tissues, including the brain, bones, liver and joints.

In addition, drug-resistant TB is becoming more common, in which the bacteria no longer respond to one or more of the antibiotics used for treatment.

Other diseases with lower infectivity include Ebola - which has a very high fatality rate but is transmitted through close physical contact with bodily fluids. Its R0 ranges from 1.5 to 2.5.

The lowest R0 values - below one - are seen with infections including Middle East respiratory syndrome (Mers), bird flu and leprosy. Although these conditions do not spread as easily, their seriousness and the complications they can cause should not be underestimated.

The risk posed by any infectious disease is shaped not only by what it does to the body, but also by how efficiently it passes from person to person.

Preventative measures such as immunisation are crucial - both for protecting individuals and for reducing spread to people who cannot receive certain vaccines, including infants, pregnant women, and those with severe allergies or weakened immune systems. These groups are also generally more susceptible to infection.

This is precisely why herd immunity matters. When immunity is widespread across a population, it helps shield those who are most at risk.

Dan Baumgardt, Senior Lecturer, School of Physiology, Pharmacology and Neuroscience, University of Bristol

This article is republished from The Conversation under a Creative Commons licence. Read the original article.