Exposure happens through contact, contaminated surfaces, and the air. Even after the visible water is gone, pathogens remain in porous materials and on surfaces until they are properly removed and disinfected, which is why a mop-and-bucket cleanup is not enough.
Some people are at higher risk - young children, older adults, pregnant people, and anyone with a weakened immune system or a respiratory condition. For them especially, staying out of the affected area until it is professionally decontaminated matters.