What makes a person asymptomatic? How does it work?

Question

As someone who's simply asking out of curiosity, I've tried looking into the topic myself, but often times I run into answers with little or no detail on the subject. Sometimes the question is more focused or directed towards something else in the system.

What I want to understand isn't the effects of being asymptomatic, but rather, how does the whole process work? Why is it different person-to-person, and what exactly is going on behind the scenes at a biological level? Does one person just have a stronger immune system able to suppress the virus longer? Or a more reactive Immune system that quickly tackles the virus before it can proliferate to threatening levels? Perhaps the virus itself is still somewhat dormant or not fully awake yet?

In short, what EXACTLY causes one to be asymptomatic and in turn, become a carrier rather than someone who looks like a walking corpse?

Answer 1

The symptoms caused by a virus come from a combination of the actual damage done by the virus and side effects of the immune system's response to the virus. Once a virus enters your body there is effectively a race between the reproduction of the virus, and the immune system developing a targeted response to the virus. Also going on in the background, the body is repairing damage to itself. It's also possible that the body's immune response will become too intense, and this can kill cells too.

So why wouldn't a person develop symptoms from a virus infection? Well, this is biology so individual variation counts for a lot. To infect a cell a virus has to dock with proteins on the surface of the cell. All humans will have basically the same surface proteins, but there may be slight differences from person to person. In some individuals the differences may be sufficient to slow down the virus's entry or cause it to fail entirely sometimes. This would slow down the virus life cycle, which would give an extra time for the immune system to react to it and knock it down, perhaps before either the infection or the immune response became intense enough to cause symptoms.

There is considerable variation in our immune systems as well. As you might guess age, or medical conditions may result in a weakened immune system that responds to the virus only slowly. Our immune systems are also changed by the diseases we've encountered in our life. For example, some people may have encountered other viruses in the same family as SARS-COV-2 and have stored anti-bodies that are partially effective against SARS-COV-2.

Some viruses like HIV and herpes can undergo a period of dormancy.

The most famous case of an asymptomatic carrier of a disease was Mary Malon who was infected with typhoid fever. Though she displayed no symptoms, she transmitted the disease to dozens of people, several of whom died. She was forced to spend 26 years in quarantine.