I bet he passed out and forgot to lock the door, or left the door unlocked for megumi and went to bed, and hanazawa went to check on him, and went inside when he didn't answer the door.
@thedude
But the way people usually get colds in manga, and the fact that they tend to shake it off after a day or two, seems to indicate that they are, indeed, supposed to be colds.
Seriously, in at least one story someone gets a cold because they accidentally had some cold water poured on their shirt, and only their shirt, and only part of their shirt, and they were inside and could easily have dried it off within a matter of minutes.
Apparently, manga artists are convinced that humans don't have immune systems. Cells at Work must have been the government's attempt to teach them otherwise.