It's because usually when there's more people in confined area, not everyone is taking the proper procautions like washing their hands,  or covering their mouth when they cough or sneeze.

Massive crowds of people indoors in close proximity, creating an environment where germs stick to surfaces.  Vitamin D, which helps build the immune system, may be lower due to less sunlight. The laws of thermodynamics, means cold air can carry less water vapour before it reaches the “dew point” and falls as rain. So while … Read more