Cold and Flu, are common viruses which normally infect us in the cold of winter. Ironically, those people who always keep themselves warm, stay less out-of-the-house and office, keep their windows shut, and don't venture out beyond essential travel seem the most affected.
I usually prevent those infections from exploiting temperature drops in the body by allowing myself to adjust to the gradually-declining temperatures in the fall. I do dress-up more in winter. However, I stay-out more than others, I don't dress as heavily if I stay out for a short while (An hour or less) or if temperatures are not too low. I take my cold showers year-round and I keep my room ventilated. cold showers train the body to maintain normal body temperature above that comfortable for such viruses. At this time of the year, I seem to avoid the cold and flu when people have it most.
My tendencies to have these common viruses are:
When I'm in the proximity of a sick person.
When the spring temperatures swing wildly and I cannot find a comfortable level of protection without sweating too much while facing a cold breeze.
When I'm well dressed, but keep getting cold air on my face. Think of standing under the A/C in summer as an example. That's my #1 reason to avoid sitting beside an A/C vent. Different body parts are exposed to different temperatures and body thermoregulation is more challenging. The virus exploits colder body parts as its backdoor entry into the body.
For the purpose of discussing enclosed space habitats, let's take-out the 2nd and 3rd option, and deal only with the first one. You may also deal with the A/C scenario in the third case.
Assume the space colony is enclosed and air is recycled, i.e. the outside conditions mandate it. Orbital space-colonies have the same problem and have to deal with hygiene challenges as well. All habitats, whether in orbit or on the surface of a planet, have contact with the outside world and accept many visitors, therefore increasing chances to introduce a sick person on board.
EDIT: A space colony may have the population of a small village. Assume the range of people sharing the inner space starts-out with 50 individuals. They take part in physically building the colony before introducing a breathable atmosphere into the system. The colony may be large enough to accommodate 200 people and an extra space for 50 visitors at any time. Space travel has just begun to become popular, so square-foot per person is limited. There's just over 200 square feet average for an individual or a small family for comfort, sleep and hygiene, and a self-service lunch room is shared with others. The square-footage for space stations in orbit may be down to 100 square feet to mostly singles and young couples.
Given these unfavorable (for us) conditions, the common viruses have a year-round amiable conditions needed to spread, and recycled air's quality is not quite the same as that of fresh air. Is that a chance for common viruses to attack more often? to develop into more violent strains, such as pneumonia or worse?