Why do people get sick in December and January?
a) That would depend upon whether a person lived in the Northern hemisphere or the south and in which country and the location within the country. b) Northern Hemisphere in countries and locations that are cold/snowing etc would mean becoming unhealthy moving from cold outside to warmth inside a home then back outside again. Poor ventilation, stuffiness, lack of Vitamin D from lack of sunlight etc. c) Southern Hemisphere -- Hayfever and Asthma from dust, pollen and other allergies in Spring/summer weather.
Maybe the weather
Being inside in nice warm, moist conditions with poor ventilation, little exposure to UV light, your immune system working flat out, germs breeding very well ...
The environment becomes a hardship on our bodies, thusly weakening the immune system. Various viruses also spread best at certain temperatures.
Most of the country is cold and with that sniffles and then gets worse.