Virus is the cause.

 Can you catch a cold from walking into the AC after working up a sweat?

Everyone is familiar with the common cold. Young children get five-to-seven a year, and adults two-to-three.  But there's also a familiar saying that is spread from parent to child, and from generation to generation about colds.

Viruses are spread when an infected person coughs or sneezes near your face, or when you touch the virus on a surface or someone's hands, and then drive it in your eyes, nose, or mouth, from your hands.

The cold temperature doesn’t make you any more susceptible to colds. Infections again, the responsibility for the infection itself is a virus. It's not the temperature itself.  

The virus may travel in cold, dry air better from a cough or sneeze. And when your nasal passages are cold, immune cells might not work as well.  And as a result, the viruses, if they're present in the nasal passage, may replicate more effectively.  But the virus still has to be present to make you sick.

Remember it's not the temperature, but it's the spread of the virus that makes you sick.