It‘s the other way round, they sound like a perfect reservoir for viruses. They might have benign viruses on top of the deadly ones in their system. Why should there be a selection for deadly ones if they are a great reservoir for virus amplification?

Putting it very simplistic: it is all about (physical) chemistry. Molecule X binds to a receptor, which changes its form, which triggers another reaction, etc.

It’s a huge chain reaction with some probability distribution (depending on binding strength, temperature, etc.)

Also not blood transfer, but there are cases that children acquire cancer from their mothers during birth.

Here is one example and the mention other cases in this article as well: https://www.upi.com/Health_News/2021/01/07/Women-can-transmit-cancer-to-infants-in-childbirth-reports-suggest/7101609975411/

A good example for those crossover design are the Covid vaccines. A friend of mine took place in the test of the vaccine. She was on the control group and was offered the vaccine after the end of the trial.

No, the incidence you mentioned was not a double-blind study. It was a Phase I clinical study. The first test of a drug in humans. This test is done to test the safety, the side effects, the best dose and the best formulation method for the drug. All probands in a clinical phase I study are healthy individuals without the disease in question and all of them receive the drug (after intense studies in animals).