Why does the mortality rate of an infectious disease decrease after a period of time?

There are many similar situations in historical records. When a certain infectious disease is prevalent repeatedly, people's symptoms will become milder and the mortality rate will decrease.

The reason for this phenomenon is often not that the pathogens of the infectious disease have reduced their transmissibility and pathogenicity due to genetic mutations, but its essence is more likely to be the result of natural selection in which infectious diseases eliminate susceptible individuals in the population.

In fact, humans, animals, bacteria, viruses and other life forms have always evolved under the premise of constantly adapting to the environment. The birth, aging, illness and death of humans, animals and plants can actually be understood as a metabolic phenomenon in the process of biological evolution in nature. Viruses are a carrier or catalyst in the process of biological evolution: viruses will kill many infected species, and the surviving species will then form antibodies to the virus, so the evolution of species is completed in this way.

Let's take it a step further: How do animals resist viral infection? If we temporarily put aside the social attributes of humans and understand humans as an animal (a higher animal) in a purely biological sense, the game between all animals, including humans, and viruses has begun since the birth of the first life on this planet. It's just that we humans have our own unique medical system when facing viruses, while ancient humans and other older species have been fighting against viruses for billions of years before that.

In fact, we are exposed to a lot of bacteria and viruses in our daily lives: in fact, hundreds of millions of viruses fall on every square meter of land every day. This sounds like a terrifying thing, but in fact it is not as scary as we imagine. The evolution of organisms itself is a process of continuous adaptation to the environment: since the living environment of all animals, including humans, is full of bacteria and viruses, animals will naturally gradually adapt to this environment during the evolution process. All animals have evolved a set of immune systems for the bacteria and viruses they face in daily life. In fact, our bodies are fighting against bacteria and viruses that invade our bodies all the time: first of all, our skin will be the first line of defense against bacteria and viruses. If this defense level is lost, the immune cells in our blood will eliminate the invading bacteria and viruses. In case this second level is also lost, then we still have a third level - this is our immune system, just like the vaccine we take, it will produce targeted antibodies and eliminate them immediately once they are found. Most of the time, the immune systems of animals, including humans, can inhibit viruses. In fact, most animals, including humans, carry quite a few viruses, but they do not become sick because of carrying these viruses. This is because our bodies already have natural antibodies against these common bacteria and viruses in daily life.