The Human Microbiome is the first line of defense. There are so many Microbes, some good, some bad, some neutral, covering the body and in the digestive tract that they should be considered an organ. Due to competition among themselves for nutrients, they can fight off invaders. Then the Innate Immune System (we are born with it) kicks in and like a battle, chemically detects invaders, it can bind to pieces of the invader and send signals to other cells who deploy weapons which can kill the invader or kill the cell that has been invaded. Some cells can heroically commit suicide.

If that fails, then the Adaptive Immune System is activated which involves T cells from the Thymus and B cells from the Bone Marrow. They can detect the specific invader and they learn to defend against it. The cool thing is that they remember that particular invader so the Innate System doesn't have to go through all its stuff and waste time. Some invaders can live nicely on extracellular nutrients and others like viruses are Obligate Intracellular Parasites and have to invade the cell to reproduce.

The invading microbes have their own defenses. They can evade detection by grabbing pieces of the host. Some have up to 60 genes that can encode different 'coats' so once they are detected and the adaptive system is engaged, they literally put on a new coat. Others can invade cells that aren't well protected and some can kill the immune weapons that are deployed. There are diseases that have no vaccine and are endemic like sleeping sickness and malaria. The flu, instead of camoflaging itself, mutates slowly so vaccines are only good for one year. And there are more drastic ways microbes defend themselves by sudden antigenic shift or recombination, such as the H1N1, H5N1, H7N9 viruses. Be very afraid of Bird Flu.

Some vaccines do provide life-long immunity or close, such as polio, measles, chicken pox, although that virus learned how to hide in nerve cells and can reappear to cause chicken pox in the unvaccinated and shingles in the host. (Take the new Shingles shot, Shingrix)

Some diseases are regional or seasonal. Mutations are sometimes discovered this way by it appearing in the wrong season or area. Some pathogens invade and then disappear for a time because they have a different vector host.

Host resistance is affected by nutrition and genetics. For example, two mutations on the same gene can result in sickle-cell anemia. However if you have the mutation on only one of the genes, you have resistance to malaria. But those people are less resistant to the flu, so there is no free lunch. Immunity is even mediated by social factors, family, friends, instrumental support, emotional support, yet there are diseases that have a stigma and will result in avoidance, stress, and depression, which adversely affects the immune system.

Antibiotics do kill beneficial bacteria so it is a good idea really to limit taking them unless you cannot fight off the illness. Plus there is so much resistance that there are only a few antibiotics left to treat them.

There are many factors which determine if an infection will be local or an epidemic.

There has never been a working vaccine for any Human Coronavirus so I am waiting to see what they come up with. It is so interesting!!!
[Post edited by TechMomof2 at 07/07/2020 12:21PM]