In terms of infectious illnesses, germs make you sick, not cold weather itself. You have to come in contact with rhinoviruses to catch a cold. And you need to be infected with influenza viruses to contract the flu. Rhinoviruses peak in spring and fall, and influenza viruses peak in winter
Under cold conditions the organisim will be under cold stress that will affect immunity thus the organism will be easly infected with the viruses and other infecting germs.