Why didn’t things like polio and smallpox mutate as much or as dangerously?

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Why didn’t things like polio and smallpox mutate as much or as dangerously?

In: Biology

They did. It’s just that mutation rate is proportional to the number of people who are simultaneously infected, and the mutations went relatively unnoticed due to technology not being as good back then.

Different types of viruses mutates at different rates. Viruses like Rhinovirus and Coronavirus is known to mutate very fast which helps them avoid the immune system. But the changes means they only last for a short while, maybe just two years, before being out-competed by a new variant of themselves. However viruses such as polio and smallpox have a much more protected way of reproducing which does not cause as much mutations and we can therefore find largely unchanged viruses a hundred years later.

I do not have the exact answer, but I can provide quite a few generally valid reasons.

First, evolution is done by being randomly different from a previous generation. Then surviving long enough to have a new generation and pass it on. For animal, it’s pretty easy to understand, you live and reproduce, or your mutation dies with you.

For diseases though, it’s a bit more complicated. They don’t live alone. They NEED a host to live. Which mean they need quite a few condition to properly mutate.

1) Have a host survive long enough to have more generations. The more generations, the better (can be achieved by either not killing the host too fast, or reproducing fast. Or a combination of both)

2) Be able to transfer to a new host. It doesn’t matter if your new variant is super powerful and can wipe humanity, if nobody catches it before it dies, it’ll cause no harm.

Polio and smallpox were taken very seriously for centuries. Disease were things that could wipe out entire villages. But this made it difficult for these disease to fill the condition of mutating succesfully. Couple that with medicine, and it’s almost impossible.

Have an outbreak in a densely populated and developed area? Chances are you’re gonna get proper treatment, and isolation, preventing the disease from spreading.

Have an outbreak in a small village where they don’t have a doctor available? The village will die and nobody will have the disease anymore.

Covid is pretty much at the best situation: Careless people, lot of travel over large distance (so no shortage of hosts), fast reproduction, No early symptoms allowing it to hide long enough to get new host before the original even knows he’s infected. Our lifestyles have changed drastically over the last few decades, and they were no longer adapted to protect each others from disease.

They are DNA viruses with more robust self repair mechanisms.

DNA is inherently more stable than RNA because its structure is physically stronger.

Self repair mechanisms delete many mutations before they have the opportunity to propagate.

Different organisms mutate at different rates. Turtles haven’t changed much in hundreds of millions of years. Humans developed from an ape-like ancestor just 2 million years ago. Some coronaviruses are extremely prone to mutations – hence why no vaccine for common cold caused by them as it’s almost never the same strain or combination of strains that infect you every year.