If you work in an office, it might be hard to hear yourself think these days, what with the chorus of coughs chiming from every direction. It’s a time of year when simply staying healthy seems like a herculean task.

Most of us focus on trying to get enough exercise and sleep, by getting a flu shot, and by fastidiously washing our hands. When it comes to common colds, there’s only so much one can do. But with other communicable diseases, we often have an invisible protective shield.

We’re part of the herd.

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We — and this is most especially true for the immunologically vulnerable among us — benefit from the effect of herd immunity. The concept, also explained by STAT’s Alex Hogan in the animation above, is a pretty intuitive one. Communicable diseases that would spread easily within a population of people who are universally susceptible are much less likely to find and infect the few vulnerable people if most of the population is immune to them.

That immunity can be acquired through infection or vaccination. In the case of measles, for instance, most people born before the 1970s became immune the hard way, by suffering through a bout of the supremely infectious and miserable illness. Children born in the era of the highly effective measles vaccine have it to thank for their protection.

But within any group of people, there will always be some who are not protected: tiny infants too young to be vaccinated; the elderly, whose immune systems are succumbing to the ravages of age; people undergoing cancer therapies; those born with immune system deficiencies or who developed them later, through infection with HIV.

These vulnerable people hide within the herd, benefiting from the protective barriers it throws up. But if the herd thins through vaccination refusal, diseases that had been previously kept at bay can start to threaten again.

A case in point is the current measles outbreak in New York and New Jersey. The virus was introduced into a community of ultra-Orthodox Jews, who eschew vaccination. The ensuing outbreak is one of the largest in the United States in decades.

There’s an important caveat to think about, one that explains why health professionals ask so frequently about tetanus vaccines.

Herd immunity only works for diseases that spread from person to person. But some diseases are not spread that way — we contract them from encountering a pathogen from our surroundings. Tetanus is one of these.

Everyone in your neighborhood — in your entire city — could be protected against tetanus. But if you forget to get a tetanus booster shot and your tetanus protection wears off, you will develop tetanus if a nail you step on while gardening is contaminated with spores of Clostridium tetani.

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