[bob47]

I'm hoping some folks with expertise in immunology can shed some scientific light on the spread of diseases like the coronavirus.

Is there a "critical mass" of the virus that an individual must be exposed to to become infected? In other words, can some people be exposed and escape the disease?

What are the factors that determine how severe of a reaction a person will have when infected?

Does having the disease, or a vaccination in the future, give lifetime immunity or will this thing evolve like the flu and become something else that we must be vaccinated for every year?

Thanks for any information that helps us understand this better.