Most readers of this blog are familiar with "SchrÃ¶dinger's cat". The idea is that if you put a cat and something that could kill it into a box, you won't know whether the cat is dead or alive until you open the box and look inside. It's a thought experiment meant to demonstrate the problem with trying to measure quantum mechanics. The idea is that the act of observing (or measuring) something can change its state, thus making the observation measurement flawed. This notion can be applied to measuring electrical current, temperature, etc., although to a much lesser degree.
How does this apply to tRump? Monday, tRump declare that he had been taking hydroxychloroquine for about 10 days. That would correspond to the date when it was announced that tRump's personal valet had tested positive for Coronavirus. He stated subsequently that he was taking it as a "preventative".
Never mind that tests show that it's efficacy is nil, and positive results are strictly anecdotal. The few somewhat controlled studies have showed no difference in rates of improvement between the drug and a placebo.