i'm not sure if there is a "right" -- it's another one of those areas that's filled with controversy.
my doctor, whom i trust 1000%, says that lifting can make an illness worse, as you over-tax your systems, especially your immune system, and it's a good idea to cut out the weights during an illness.
however, low to medium intensity cardio can be good for you as it gets the circulation going and if you have any kind of congestion it can help that too.
i've followed his advice on this subject these past 8 months and i can say that i've found it to work for me. however, i have not followed his advice about my carpel tunnel and now i'm afraid to go back to him and tell him that i'm in more pain
i think it's all a matter of "how sick" and "can you really do something that's worthwhile?"
sending you all healthy vibes
adriana