“There is no question but that perfect sanitation has almost obliterated smallpox, and sooner or later will dispose of it entirely. Of course, when the time comes, in all probability the credit will be give to vaccination.” — Dr. John Henry Tilden, M.D., author of Toxemia Explained

Everyone has been gaslit into believing the actual cause of those diseases is a virus, but if you do serious research into what is known about any specific virus, you'll find that it's never been isolated or shown to cause any disease. You'll also find out the real cause of the disease, and the ridiculous methods used to diagnose it, which will explain everything that is observed about it.

As for diseases which are usually only contracted once, exposure to the conditions which are needed for them to manifest are just rare, and you'll also find that reoccurrences are not nearly as unlikely as you have been led to believe.

Disease is caused by deficiencies, parasite infections, and cells damaged by toxins.

I've seen it theorized that cell damage could happen naturally as part of certain developmental processes (chickenpox). I wouldn't rule out chickenpox being a parasite though, since encysted parasite lava would explain why people get Shingles later in life when their immune system is compromised. More research is needed.

Deficiencies impair immune functions, and can even be caused by all kinds of serious stress which suppresses the immune system so cells don't regenerate.

Toxins keep the immune system busy, and that makes your body more susceptible to parasite infections. That includes common things like Flouride and metals in water, Glyphosate in food, Aluminum in the air "to fight global warming" and also in vaccines. Aluminum makes you more conductive to EMF pollution, and allows parasites to pass the blood brain barrier and enter your brain. Parasites also create an environment where your own cells produce toxins which poison you and suppress your immune system.

Even essential vitamins and minerals like the iron can poison you very badly. A common way of isolating excess toxins from the blood stream is by making fat. That's why all countries which mandate iron fortification of food have obesity epidemics since that was introduced, but neighboring countries with identical diets have no health issues. You burn more fat in cold weather, which is why you release more stored toxins and have "flu" symptoms in "flu season".

In the most extreme cases of poisoning, when the immune system can't get rid of the toxins fast enough, there are severe symptoms like measles or shingles as toxins are pushed out through your skin which has the largest surface area.