There is one general rule for illness and fitness
THE NECK CHECK
Where are your symptoms coming from? Symptoms above the neck such as a running nose, tickly cough and sneezing are okay to continue exercise with. These should not impact your routine, although you may wish to lessen the amount of time you exercise for. However, post workout if you feel worse, then the next day you should skip your workout and allow your body to start recovering.
If your symptoms are below the neck, stomach problems, congested chest, muscle ache etc then you should definitely not exercise. Exercising will make your symptoms worse, and will hamper your chances of recovering quicker. Not only could you harm yourself, you could also be contagious and affect other gym users. As a gym user yourself you will appreciate it's not nice to have ill people spreading their grerms around you. With the flu, a contagious state can last for 5-7 days.
The good news is, people who have a high state of fitness generally recover much quicker than those who don't partake in exercise regularly. Not only this, but symptoms have been shown to be weaker, and not affect the body so much.
So what's the verdict? Think about your illness carefully, where is it affecting you? Conduct the neck check, but most importantly worry about yourself more than your exercise regime.
If you are coming back from time off, make sure you ease back into your workout. Start back at 70% and work your way back to your full fitness regime over a few days, sometimes illnesses can leave us weaker than we expect!