I read like 10 replies and no one said that cleanliness and germs/viruses are not related. Sure, good practices like covering your mouth and washing your hands can help prevent the transmission of viruses, but sick people incubate millions of them and one sneeze can make an airborne virus fill an entire room - less if they cover the sneeze, but that can still effectively cover a much smaller area. Plus, you don't have to sneeze. Just breathing in and out air that is full of airborne viruses will infect others.

Point being: Cleanliness can help you not transmit diseases to your family when you share large spaces with few people, but it will not help in crowded situations.