eli5: How was smallpox eradicated but other diseases are significantly harder to completely get rid of?
In: Biology
Smallpox had a good vaccine for it which, when administered widely eliminates the diseases in all humans in that area.
In addition, smallpox has no animal hosts, so it can’t hide in other animals and eventually come back after some mutations or when we stop vaccinating.
Smallpox was quite simply a perfect target for eradication.
The biggest reason why we could eradicate smallpox is because the smallpox virus could only infect humans. Thus, once we immunized the entire human population against it, the virus completely died out. Since there were no more live populations of the smallpox virus in the wild, there were no opportunities for the smallpox virus to mutate to get around our vaccines. Today the only living samples of smallpox are kept in a small number of highly secured research labs.
Most other infectious pathogens like viruses and bacteria can infect other organisms than humans. Ebola naturally infects bats. The flu virus can infect a whole bunch of animals. These populations of viruses infecting animals thus can continue to mutate and reproduce, eventually rendering existing human vaccines useless.
You have the question backwards, but others have already covered the answer to the proper question (“why was smallpox so easy to eliminate?”) Instead, I want to use an example of a disease we’re *close* to eradicating: Poliovirus.
Polio has three main variants (wild poliovirus 1, 2, and 3). We’ve eliminated WPV2 and WPV3. The only way those viruses still exist is that, because of the “inactivated” oral version of the vaccine, it’s possible for the inactivated viruses to mutate and regain their ability to cause harm, *if* the local population doesn’t maintain a minimum level of vaccination. (As long as ~90% of a population is vaccinated, it’s effectively impossible for the virus to spread in a way that can cause damage.) This oral vaccine is useful because it allows contact immunity–people who don’t get the oral vaccine directly can acquire it from someone else who got it, which makes vaccinating large areas significantly easier (plus, the inactivated poliovirus is much easier to make, store, distribute, and deploy than the injected version.)
We’ve phased out the WPV2 oral vaccine completely, and it’s possible we might phase out WPV3 as well. Only a few dozen confirmed polio cases occurred in 2023, and if we’re lucky, we might be able to declare all three types eradicated (outside of the rare re-activated oral vaccine cases), at which point we could switch purely to the injected vaccine.
The key difference with polio, like smallpox, is that it ONLY affects humans. This is a human-only virus. If we know nobody’s gotten it for 20 years, we can be pretty confident there isn’t any of it left out in the world. It also helps that, as stated, the oral polio vaccine is cheap to make, store, distribute, and deploy.
in addition to what others have said, smallpox has a short incubation period and was easy to diagnose. Ie. you find out pretty soon after getting it that you have it, and it’s obvious you have it, making it harder to unknowingly spread than some diseases.
Much of it is a people problem.
Smallpox was literally just that bad that at the height of the Cold War both the Soviets and Americans worked together on eradicating it.
The human part is that none of the other diseases are as bad so we aren’t as motivated to cooperate. For example: we traded 5-10 years of progress on Polio eradication to get Osama Bin Laden.
I’ll add another thing that nobody else has mentioned: survivorship bias and lack of personal experience with the diseases. In many cases now it’s been 50+ years since we’ve begun vaccinating for a disease (like measles), so most everyone alive either never had contact with it, or was a small child when they got it and was unaware of other children that died of it. They simply remember being sick and then getting better. Most people old enough to remember their children dying of diseases like measles aren’t around anymore so we have a population that has no idea of how devastating these illnesses can be, that thinks “well I survived it so it can’t have been that bad” (you see them saying the same thing about seatbelts in cars). So it becomes much easier to convince people that these diseases weren’t actually a big deal and that vaccines are actually worse/more dangerous than getting the disease.
Another factor: Smallpox is a DNA virus. It has, compared to things like the flu or COVID which are RNA viruses, a quite stable genome. It doesn’t mutate very fast at all, and modern studies in the small bits of smallpox virus that still exist show it wasn’t particularly diverse, so it didn’t really have any tricks up its sleeve to escape from the vaccine when we came for it unlike influenza viruses, which mutate so fast that we need yearly vaccines.
Big Pharma won’t make money off of the cure, but in managing the pain associated with the illness/disease.
Smallpox has a relatively long incubation period of 7 to 17 days. And you weren’t infectious until the fever started. This gives your immune system time to mobilize and squash the infection before you become infectious to other people. It still takes time for a vaccinated person to get their immune system geared up to fight. It’s just that if you’ve already been exposed to a pathogen it takes ~5 days.
The first time your immune system encounters a new pathogen it takes a little over two weeks. In that time the adaptive immune system creates new antibodies and starts manufacturing a bunch of them. If you’ve been vaccinated the antibodies are already developed and the body just has to churn out enough of them to control the infection. At least that’s one level of what’s going on.
COVID-19 you’re infectious in just a day or two of being exposed and often have infected another person before the ramp up period of your immune system is over.
I’m pretty sure the eradication of small pox was like a multi billion dollar mission. Literally every small village, town and city were making you get small pox vaccines
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