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Why the Deadliest Virus Was the Easiest to Kill

2026-08-28 · explainer · Hank · 10:48 · watch on YouTube

Topic: Health · mode: Explainer

Hank explains why smallpox, though very hard to eradicate, was an ideal candidate (no reservoirs, stable antigens, low infection rate, high incentive, post-exposure and heat-stable vaccine), why TB, flu and measles are harder, and argues that diseases we cannot eradicate can still be treated.

What is said

The parts of the video and what is said in each (★ — how much it matters to the video).

00:00:00 — Frustration that people still die of preventable infectious diseases; smallpox eradicated, other diseases eliminated regionally; why not eradicate them all?

  • ★★ Hank finds it frustrating that people still die of preventable infectious diseases in a world that has proven an entire disease can be eradicated. — 00:00:00
  • ★★ Hank says eradication was proven in 1980, 46 years ago, joking that he knows because that is the year he was born. (playful) — 00:00:00 · Hank Green
  • ★★★ Hank says smallpox is the only human infectious disease ever entirely eradicated, though polio and guinea worm are within striking distance. — 00:00:29
  • ★★ Hank says rubella has been entirely eliminated from the Americas. — 00:00:29 · United States
  • ★★ Hank says malaria has been eliminated from the US, Europe and parts of Central Asia. — 00:00:29 · malaria, United States, Europe
  • ★★★ Hank says more could be done and some eradicable diseases have not been eradicated, but some diseases cannot be eradicated, since pathogens differ in how easy they are to eradicate. — 00:00:58

00:01:27 — Smallpox as an ideal candidate; reservoirs: latent TB and influenza A in animals

  • ★★★ Hank stresses smallpox was very difficult to eradicate and took coordinated, high-quality human effort, but it was also a somewhat ideal candidate for eradication. — 00:01:27

  • ★★★ Hank says reservoirs, places a disease can hide and then come back from, are probably the most important factor in whether a disease is technically eradicable. (hedged) — 00:01:27

    “places a disease can hide and then come back from”

  • ★★★ Hank says smallpox had no reservoirs: it exists only during illness, after which the patient dies or the immune system clears it, and only humans get it. — 00:01:56

  • ★★★ Hank says tuberculosis has a huge latent human reservoir: even in the US, where TB is rare, something like 13 million people carry TB bacteria without symptoms. (hedged) — 00:01:56 · tuberculosis, United States
  • ★★ Hank says latent TB is almost certainly not transmissible, but it can stay dormant for decades until the immune system falters, then becomes transmissible disease. — 00:02:26 · tuberculosis
  • ★★ Hank says eradicating TB would mean not just treating active cases but putting out fires for decades as latent infections emerge. — 00:02:55 · tuberculosis
  • ★ Hank says TB also infects animals, but animal TB transmissible to humans seems somewhat different, so the animal reservoir matters less than the latent one. (hedged) — 00:02:55 · tuberculosis
  • ★★★ Hank says influenza A has very active animal reservoirs, infecting everything from seals to seagulls and circulating constantly among waterbirds, so it could return from a duck even if cleared from people. — 00:03:24

00:03:53 — Antigens: smallpox's are stable, flu's change every year

  • ★★ Hank explains antigens: the immune system identifies pathogens by molecules on their surface, learns to attack them, and those molecules are called antigens. — 00:03:53
  • ★★★ Hank says smallpox's antigens are very stable, so people got it only once and a vaccine could give very long-lasting immunity. — 00:03:53 · vaccine
  • ★★★ Hank says flu antigens constantly swap, evolve and change, which is why one smallpox vaccine sufficed but flu needs a different vaccine every year. — 00:04:22

00:04:52 — Measles: infection rate, vaccination thresholds and the lower incentive because it is less deadly than smallpox

  • ★★ Hank says measles is pretty eradicable because, like smallpox, it is antigenically stable and has no animal reservoirs. — 00:04:52
  • ★★ Hank says measles was declared eliminated from the United States in 2000, showing it can be eliminated from the whole world. — 00:04:52 · United States
  • ★★ Hank says measles is harder to eradicate than smallpox for two reasons: how the disease itself acts and how people react to it. — 00:04:52
  • ★★★ Hank says each smallpox infection caused three to six new infections on average; once that number falls below one, the disease dies out. — 00:05:21
  • ★★★ Hank says one person with measles infects 12 to 16 others on average, a tremendously high infection rate. — 00:05:50
  • ★★★ Hank says bringing smallpox below one might take vaccinating about 80% of exposed people, while measles needs something like 95%, which is much harder to reach. (hedged) — 00:05:50 · vaccine
  • ★★ Hank says one dose of measles vaccine gives 93% immunity and two doses, given some time apart, bring the population to 97%. — 00:06:19
  • ★★ Hank says measles keeps transmitting if just 5% of people are hard to reach or refuse the vaccine, so only places with strong public health systems can eliminate it. — 00:06:19
  • ★★★ Hank says measles's second advantage is more our fault: it is not as bad as smallpox, so the personal incentive to vaccinate is lower. — 00:06:49
  • ★★ Hank says measles is awful, kills people and children, and should be eradicated. — 00:06:49
  • ★★★ Hank says smallpox had a death rate as high as 30% and survivors often had lifelong facial scars, so its toll was visible and the incentive to vaccinate extremely high. — 00:06:49
  • ★★ Hank says even where health care was bad, measles death rates were about 10 times lower than smallpox's. (hedged) — 00:07:18

00:07:47 — Smallpox's further advantages: post-exposure vaccination, contagious when sick, distinctive symptoms, a vaccine that needs no cold

  • ★★★ Hank sums up the biggest advantages against smallpox: no reservoirs, stable antigens, a relatively low infection rate and an extremely high incentive to avoid it. — 00:07:47
  • ★★ Hank says smallpox vaccination helps even after exposure, so vaccinating everyone around a case reduces transmission in the second wave. — 00:07:47
  • ★★ Hank says smallpox patients were most contagious when most sick, unlike COVID and measles, which spread before people know they are sick. — 00:08:16 · COVID-19 pandemic
  • ★★ Hank says smallpox had very specific symptoms, unlike diseases that can be asymptomatic or mild in some people. — 00:08:16
  • ★★★ Hank says scientists learned to keep the smallpox vaccine active without cold, which made worldwide delivery much easier; many vaccines still need cold. — 00:08:45 · vaccine
  • ★★ Hank says smallpox eradication was very hard and we should take the win; he is proud of us. — 00:08:45

00:09:15 — Diseases we cannot eradicate can be treated; thanks to supporters of PIH and Save the Children; predictions on polio and flu; sign-off

  • ★★★ Hank says a disease that cannot be eradicated can still be treated: many treatable diseases go untreated, and we can make far fewer people and children die of infectious disease. — 00:09:15
  • ★★ Hank says the only way that work gets done is if we care about it. — 00:09:15
  • ★★ Hank thanks everyone who has supported Partners In Health or Save the Children over the years; much good has been done. — 00:09:44 · Partners In Health, Save the Children
  • ★★★ Hank says smallpox was among the five worst human diseases, but its qualities made it a good candidate for eradication, so we now live in a better world. — 00:09:44 · progress
  • ★★ Hank is fairly confident humanity will eventually eliminate polio. (hedged) — 00:10:20
  • ★★ Hank is confident, with some doubt, that influenza will not be eliminated: in his lifetime we will still get the flu. (hedged) — 00:10:20

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Mentioned

Central: Antigen · Eradication of infectious diseases · infectious disease · measles · Natural reservoir · smallpox

Also: Organizations: Partners In Health · Save the Children

Concepts: Antigenic variation · Asymptomatic carrier · avian influenza · Basic reproduction number · dracunculiasis · herd immunity · immune system · Influenza · malaria · polio · progress · Rubella · tuberculosis · vaccine · Vaccine hesitancy

Places: Americas · Central Asia · Europe · United States

Events: COVID-19 pandemic

Products: Measles vaccine · Smallpox vaccine

Also centred on infectious disease: I Predicted the Pandemic (over and over and over again) · The Great Thing about Infectious Disease

Also centred on measles: We Will Eradicate Measles · I'm Sick Because of the Social Structures of Early Hominins

Also centred on smallpox: On Immunity, Inoculation, and Individuals

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