This maybe just an isolated incident, it is the winter and people do get sick more often. When this should become a concern is if you find yourself getting constant little colds and flu's all the time.
This is one of the primary signs that your recovery is not matching your overload.
I had very similar problems with some of the programs on this site because they were too high volume for my individual recovery capabilities.
There are a lot of factors beyond genetics and how you grow up, that feed into determining your recovery ability.
Sleep (I get very little), nutrition, ergogenic aids, steriods, stress, daily activity levels at work(office vs. a lumber jack) and environment all play parts in how well you recover from your workouts.
I find that given my current situation, I can do no more than 22 total sets per workout of which few if any of those sets can be taken to failure without overstraining my CNS or causing overreaching effects on my body.
To directly answer your question, I have found 10 days after the main symptoms have cleared, should you try to give it a hard go (i.e. stimulate don't annihilate in the meantime) after such a sickness. This is very conservative but after being sick your body is working very hard to get back to homeostasis and doesn't need the competition of workout recovery to complete for it's limited resources.
You will only get sick again.
My own cure for sickness involves taking airborne 4 times a day while drinking 2 gallons of water a day.
I also try to sleep as much as possible meaning that once 9 p.m. hits I hit bed even if I am not tired. Sometimes I will have to unplug the computer and t.v. and all the lights and be pretty stern with my girlfriend that she either shuts down, goes to another room, or goes to her house so that I can sleep. I also will take mini naps throughout the day ranging from 20minutes to one and a half hours if possible.