Because disease doesn't discriminate

Holiday times are notorious for spreading germs due to close contact like hugs and kisses, or even shaking hands.

.. and I believe there is now claims that holidays are cause for higher than average illnesses due to increased sugar intake thanks to all the goodies people eat around that time

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

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.