I just got over a nasty cold. This one was a real winner. It was so bad at times I thought I had the flu. I haven't felt that miserable in a long time. Sore throat, congestion, sinus headaches, weakness, aches, chills, cold sweats, low appetite at times, a low-grade fever. After a weekend spent in bed and some ibuprofen and Alka Seltzer, I am on the mend again.
For the record, summertime colds are no fun. Colds in general are no fun, but especially not in the summer when you want to go outside.
There are some times when it is OK to run when you are sick and other times when it is not OK to run when you are sick. Ultimately, you need to weight the risks and benefits and decide which ones are greater. Running can be a good decongestant, for example. That's good. But running when you are weak could further run you down. That's bad.
The general rule is OK to run if it's above the neck and not OK to run if it's below the neck. However, there are exceptions to every rule. Go by how you feel. If it doesn't feel right, don't do it. If you are sick enough to stay home from work/school/other commitments, you are sick enough to skip running. If you aren't eating much and/or aren't properly hydrated, you probably shouldn't run.
If you choose to run:
Run outside. Skip the gym. Your gym buddies will appreciate you not sharing your germs.
Stay hydrated. Bring water with you, even if you usually don't.
Suck on a cough drop or hard candy to sooth your throat.
Don't push it or overdo it. Pushing it when you shouldn't push it could set you backwards more than resting.
Slow down. There is no award for running fast while you are sick.
If you choose to not run:
Don't beat yourself up for missing a workout. You can't always prevent getting sick.
Enjoy some extra rest. Sometimes rest is best.
The good news is I've noticed that since I've been consistently running (and mostly consistently running outside), I haven't gotten sick very much. I had one cold all last winter. One. And that was in November, coincidentally during a relatively low-mileage time. One theory is that running boosts your immune system. Another theory is that temporarily raising your internal temperature kills off germs before they make you sick. Either theory seems plausible.