The large rallies were... questionable. The large gathering in the rose garden (confirmation party?) was... absurd. It is the most likely "source".Completely agree. A virus is a contagion. Wanna go out and play? Gonna get the virus. And Trump has gone out to play more than most, and based on his own commentary, it is unlikely he has taken required precautions to prevent getting the virus.
This is far, far more likely than some conspiracy meant to take him and the White House down.
Aside from those obvious events the WH staff was purportedly tested daily. Not likely the same invasive test you or I would receive but still. Not useful, however, if you abrogate those precautions regularly.