One of the main reasons why some people prefer to work at home during the monsoon, is because they often get wet during monsoon, since the raincoat or umbrella does not keep them dry,especially during very heavy rain.
Wearing wet clothes, reduces the immunity to a great extent and makes the person fall ill.
Also many areas are water logged, dirty water from cars and other vehicles may be splashed on the person when outdoors, The dirty water often contains harmful bacteria and other microbes causing diseases.