Why does Trump's health have such a big impact on the US stock market?Shouldn't everyone expect him to have access to the best possible medical care since he is (and is also running for) president?

In the short run markets move based on market sentiment while long term follows fundamentals and valuations.

It's not so much what happens in the world rather how the market reacts to news. With election coming up, it has been historically proven evidently and statistically that the market favours incumbent party in the short run. Any uncertainty may shake up the market and cause some volatility.

However, in terms of longer term perspectives the market has on average done better over a long term with a democratic president compared to republican. These are just historical facts.

Trump is over 70 years old and obese, putting him at one of the highest risk categories for those contracting COVID-19. Even if he receives the best medical care available, there is still a risk for complications. After all, even the ultra-rich can't live forever. He has the final say on a lot of matters, and to delegate that responsibilty to others could pose significant challenges.

There are also fears that Mike Pence would contract COVID-19, who is next in line for succession. If both Trump and Pence become incapacitated, it would be House Speaker Nancy Pelosi, a Democrat, that becomes the acting President. Markets hate this kind of uncertainty and the possibility of an acting President that hails from a different political party would definitely be a huge cause for concern.​​​