read what eddy wrote above.
now read what I now write so you understand. when the body thinks its under attack is has mechanisms that it sends out. this includes adjust body temperature (known as a fever), increasing mucous production (runny nose, sore throat), etc etc. when you inject that foreign stuff into your body once again your body thinks its under attack when it really isn't.. just like how people get allergies. same thing.
if you want me to use simple terms basically what you are injecting your body is allergic to and assumes its being attacked so it sends out a response. its never a virus that makes you sick, its your OWN BODY that makes you feel sick. this is why people who reduce fevers are actually prolonging their problem, you are getting in the way of the bodies defense mechanisms and its ability to fight your virus or flush that virus away. now you can boost your own immune system to help out by taking colloidial silver, this works everytime. (of course this has nothing to do with your question but thought I would throw it in since we are on the subject, don't mean to confuse you)
and in my experience AAS will boost your immune system, i've never been sick on cycle.. I'm like a bull on cycle no matter what i run. when i come off is when i get sick some of the time.