What is Immunity?

Immunity, in my opinion, means that both humans and pathogenic (potential) microorganisms learn how to interact with each other.
The organisms that the viruses invade are strictly on a one-to-one basis and usually do not invade other organisms. However, in rare cases, this relationship is disrupted and the virus enters a previously unrelated organism. When this happens, the immune system of the organism (e.g., human) is upset by the first encounter with an unknown microorganism that it has not experienced. So, for the time being, the human body tries various methods. Viruses are generally sensitive to high temperatures, so the human body first tries to do something about it by raising the body temperature. When we suffer from a fever when we catch a cold, it is the human body's reaction, not the virus that has entered the body and is manipulating it.
Therefore, as a general rule, we should not lower the fever.
The human body tries various other defense reactions, all of which are also very hard on ourselves. Young and healthy people are fine, but those who are sick and weak cannot keep up with their own strong immune response, and in the worst case, they will die.
On the other hand, the virus, having inadvertently entered an unknown person, does not yet know what to do. Although the virus is an organism with its own genes, it does not have the ability to procreate on its own. Therefore, they enter the cells of other organisms to make them produce their own offspring. For this purpose, it would be more profitable for the other organism to live a long and healthy life. If the other party dies, the virus itself will die.
Therefore, I believe that both parties are immune to learning how to interact with each other.
Immunity is not an offensive weapon to kill "bad guys" only on the human body's side.
The next time the virus comes back, the human body, having learned about its counterpart, does not have to exhaust its strength through trial and error, as it did the first time it met the virus. The next time the virus attacks, the human body does not have to exhaust its strength through trial and error as it did the first time it met the virus.
The virus can learn to keep the other organism alive, healthy, and producing its own offspring in its body. This is the immunity acquired by the microorganisms.
Many of us have opportunistic viruses in our bodies. When they get sick, they develop bumps on the back of their lips or at the edges of their mouths. This is the work of herpes, one of the opportunistic viruses. These opportunistic viruses usually do no harm to the human body. They enter the human body when we are infants and remain dormant until we die. Of course, during those decades, they are probably producing their own offspring, but they do not do anything bad to the human body. I believe that this is the learned immune system of both the human body and the microorganisms.