Concepts explained
Can a Small Illness Prevent a Great One? — Rehearsing the Body
Country folk said the women who milked cows never caught smallpox. A country doctor believed the story enough to test it. He had no idea why it worked — there was no word for immunity yet. That method went on to erase the only human disease we have ever wiped from the Earth.
The body remembers what it has met
First, our side of the story. Piece 5 said gut neighbours train the immune system. Here is what that system does.
When something unfamiliar enters, studies its outside — the spikes on a virus coat, for instance — and builds a key shaped to grip it. That takes time. Against a stranger it takes days, and during those days we are ill.
When the fight ends, the body files the shape away. Next time the same thing arrives, the key comes in hours instead of days. It ends before it becomes an illness. That is why measles does not come twice.
The hands of the milkmaids
Jenner dug into the country saying. The milkmaids had one thing in common: they had all had cowpox, caught from the cows. Blisters on the hands, a few days of feeling unwell, then over.
The cowpox virus looks, on the outside, like the smallpox virus. So a body that remembers cowpox recognises smallpox when it comes. A mild disease taught them the face of a terrible one.
In 1796 he took matter from a milkmaid's blister and put it into an eight-year-old boy. The boy was mildly ill and recovered. Six weeks later he inoculated the boy with real smallpox. Nothing happened.
By today's standards, giving a child real smallpox is alarming. At the time there was no other way to check. The word vaccine comes from here — the Latin for cow.
Several ways to show a face
Jenner used a lookalike disease. Since then we have found other ways to show the face alone.
- A weakened original — a virus passed through generations until it can no longer make us ill. Measles and chickenpox vaccines work this way; the memory lasts long.
- A killed one — killed outright so only the coat is shown. Safe, but the memory is weaker, so several doses are needed.
- Just a piece — only one spike from the coat is made and shown. No chance of catching the disease at all.
- Just the blueprint — send the instructions for the spike so our own cells build it. Immunity sees the spike and remembers. The COVID vaccines worked this way.
That last one connects back to piece 3, where a virus was an order form using the cell as a factory. This vaccine borrows the trick — except the order form says nothing about building a virus. It describes one spike. The factory makes the spike; immunity sees it and learns the face.
Why your arm aches afterwards
The injection site swells; sometimes there is a day of fever. That is not the vaccine hurting you. It is the sound of immunity working — spotting something unfamiliar, calling in help, raising the temperature. Rather like aching muscles the day after a practice match.
You do not take it alone
Vaccines have one unusual property: they do not only protect you.
If many people in a town are vaccinated, the virus cannot find the next person to move to. With no house to receive the letter, the letter stops there. So those too young or too ill to be vaccinated are protected too. As piece 6 showed with bacteria passing abilities to neighbours, disease always travels through the people next door.
A little further in
The only disease we ever erased
In the 1960s the World Health Organization set out to erase smallpox. The method was simple: when a case appeared, vaccinate the ring of people around it and cut the paths the fire could take. The last natural case was a hospital cook in Somalia in 1977, and eradication was declared in 1980.
Smallpox was possible for reasons. It lived only in people, it showed plainly on the face so cases were easy to find, and the vaccine was sturdy. Other diseases differ: flu changes its face every year, so the vaccine must too, and some diseases also live in animals and cannot be erased from people alone.
That is the story of microbes inside us. The last piece goes outside — who made the air we are breathing right now, and why the descendants of a stranger live inside every one of our cells. The biggest story in this series.
The question that remainsIf the vaccine I take protects people I will never meet — is it a personal choice, or something we do together?