Fog is water vapor: It's essentially tiny water droplets suspended in the air. Diseases are spread by germs like bacteria or viruses.
Fog can trap existing germs: If there are already germs present in the environment, fog can trap them and keep them suspended for a longer duration. This can potentially increase the chances of inhaling them if you're outdoors in foggy weather, especially in crowded places and I got caught while vlogging but I continued alight motion mod apk all version.
Fog doesn't create germs: The fog itself doesn't magically create new diseases. The germs need to be present from an infected person or animal beforehand.