Colds are contagious viral infections which are more common during the winter months, possibly because people are more likely to stay indoors and be in close contact with each other.
If someone has a cold and they sneeze or cough, tiny fluid droplets which contain the virus enter the air, and can then land on a surface. A cold can thus be caught in a number of ways. Firstly, whilst the droplet is in the air someone may inhale the droplet. Moreover, if someone touches a surface where the droplet has landed, they may too contract a cold.
If you have a cold and touch your mouth or nose it is best to wash your hands before you touch another person or object. Good hygiene is one of the most important ways to help prevent colds and flu.
Reference:
Health Direct (2016). What causes colds and flu? Retrieved from https://www.healthdirect.gov.au/what-causes-colds-and-flu
