Why is measles one of the most contagious diseases, and what is "95 percent" about?
One person with measles can infect 12 to 18 others in an unprotected community, which is why the vaccination bar is so high.
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Measles is so contagious because the virus travels through the air and lingers there. An infected person exhales virus-carrying droplets when they cough, sneeze or talk, and those droplets can persist in the air for up to two hours. People are infectious from four days before to four days after the rash appears, and the first symptoms look like a common cold, so many pass it on before anyone knows it is measles.
Scientists measure contagiousness with the basic reproduction number, often written R0: the number of people one infected person is expected to infect in a community with no immunity. For measles it is between 12 and 18, which makes it one of the most contagious human pathogens. That is why, by the standard estimate, a minimum of 95 percent of a population should be vaccinated to stop the disease spreading among those who have no immunity. The "95 percent" is not a target someone picked; it follows from the arithmetic of a very high R0.
Measles is not harmless either. Roughly 30 percent of cases have complications such as ear infections, severe diarrhea or pneumonia. The virus can also wipe out part of the immune system's memory, an effect called that lasts for up to three years. Before the vaccine arrived in 1963, the World Health Organization (WHO) estimates, measles epidemics killed about 2.6 million people every year.
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Recap
The more people one case infects, the more of the community must be immune to stop it, and measles needs at least 95 percent.
💡 A trick to remember it · Measles is a spark in dry grass: one case lights 12 to 18 more, so almost every blade has to be damp to keep the fire out.
Surprising fact · In 1846 on the Faroe Islands, a place with no measles for 60 years, three-quarters of residents fell ill.
Connects to
- 📈 Why have measles cases returned, and what do the WHO figures show?
- 📈 What does it mean when an epidemic's R number is above one?
- 🦠 Why is the 1918 flu called 'Spanish' if it didn't start in Spain?
- 🛡️ How does your immune system remember a germ for decades?
- 💉 How does a vaccine train your body to fight a germ it has never met?
Sources (3)
No source, no claim. Every fact in this lesson (16 claims) cites at least one of these.