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From measles to leprosy: the most and least contagious diseases and the role of R₀
Last reviewed: 09.08.2025
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Each infection is assigned a value called R₀ (or "er-naught"), based on how many other people are likely to be infected by one sick person. So, with an R₀ of two, each infected person will pass the disease on to two others. They will infect four more. And so the outbreak escalates.
The R₀ value indicates how an infection will spread in a population. If it is greater than one (as described above), the disease will spread. If R₀ is one, the number of infected people will remain stable, and if it is less than one, the infection will tend to die out over time.
Circulating infections spread in different ways and vary greatly in their infectiousness. Some are transmitted through droplets or aerosols—such as when coughing or sneezing—others are spread through blood, insects (such as ticks and mosquitoes), or contaminated food and water.
If we take a step back and think about how we can protect ourselves from infectious diseases, one important lesson is to understand how they spread. And as we’ll see, it’s not just a lesson about protecting yourself, but also about protecting others. Here’s a look at some of the most and least contagious diseases on the planet.
Measles is the most contagious disease.
In recent years, measles has resurfaced around the world, including in high-income countries like the UK and US. While several factors are contributing, the main reason is a decline in childhood vaccination rates. This decline is due to disruptions like the COVID pandemic and global conflicts, as well as the spread of misinformation about vaccine safety.
The R₀ value for measles is between 12 and 18. If you do the math, two waves of transmission from the first patient could result in 342 people becoming infected. That's a staggering number from a single patient, but fortunately the protective power of vaccination helps reduce actual spread by reducing the number of people susceptible to infection.
Measles is extremely virulent and spreads through tiny particles released when someone coughs or sneezes. It doesn’t even require direct contact to become infected. It is so contagious that an unvaccinated person can catch the virus simply by entering a room where someone sick was two hours ago.
People can also be contagious and spread the virus even before they develop symptoms or have any reason to isolate.
Other infectious diseases with high R₀ values include whooping cough (12 to 17), chickenpox (10 to 12), and COVID, which varies by subtype but is typically in the 8 to 12 range. While many patients make a full recovery, these conditions can lead to serious complications, including pneumonia, seizures, meningitis, blindness, and in some cases death.
Low infectivity - high severity
At the other end of the scale, a low rate of spread does not mean the disease is less dangerous.
Take tuberculosis (TB), for example, where the R₀ value ranges from just below one to 4. This range depends on local factors such as living conditions and the quality of available health care.
TB is caused by the bacterium Mycobacterium tuberculosis; it is also airborne but spreads more slowly, usually requiring prolonged close contact with an infected person. Outbreaks tend to occur among people living together—in families, households, and in shelters or prisons.
The real danger of tuberculosis is its difficulty in treatment. Once diagnosed, a combination of four antibiotics is required for at least six months. Standard antibiotics such as penicillin are ineffective, and the infection can spread beyond the lungs to the brain, bones, liver and joints.
In addition, there is a growing number of cases of drug-resistant TB, in which bacteria do not respond to one or more of the antibiotics used.
Other low-infectivity diseases include Ebola fever, which is extremely lethal but is transmitted through close contact with body fluids. Its R₀ is between 1.5 and 2.5.
Diseases with the lowest R₀ values (below one) include Middle East respiratory syndrome (MERS), avian influenza, and leprosy. Although these infections are less contagious, their severity and potential complications should not be underestimated.
The threat posed by any infectious disease depends not only on how it affects the body, but also on how easily it spreads. Preventive measures such as vaccinations play a vital role – not only in protecting individuals, but also in limiting transmission to those who cannot receive certain vaccines, such as infants, pregnant women, and people with severe allergies or weakened immune systems. These groups are also more vulnerable to infections in general.
This is where herd immunity comes into play. By achieving broad levels of immunity in a population, we help protect those most susceptible to infection.