Do you guys want to try and start the internet conspiracy theories here? Let's start throwing some ideas around now!
Here's what I got:

Trump will not get sick and prove the virus is a hoax.

The Democrats infiltrated the White House and intentionally infected him with the virus. They possibly may have even flipped Melania to infect him.

Putin has exposed him to a nerve agent as he has no more use for his asset and it's being covered up by either the WH or the MSM or both. Putin may have used Melania as a mole to poison him.

Trump doesn't have the virus, but he is using the moment to drum up support, gain the sympathy vote, and prove by sure will he can win the election. He could also say he only lost because he was sick I guess. I dunno, this is the weakest of my theories.

And lastly, China did it.