Its chest day.. last week you were so close to putting up an all time target PB and this week you know you would kill it.. But you have the flu.. Train or not to train that is the question!
Firstly we will discuss the impact on other people in the gym... We have all seen it. The guy/girl who splutters their sniffing nose over their hands then grabs the bar for another set... no surer way to spread disease. Remember others at the gym may have young families, work as a casual, have important exams coming up and don't need to get sick by your lack of respect for other gym goers.
Your body is already working overtime trying to fight the bug you have picked up- Its worn out and you want to push it through an intense workout. Your CNS is tired from the bug, your muscles ache as you cant eat enough or drink enough to repair and hydrate them and your mind is weak with a lack of focus and energy. The risk to injury in these circumstances is clearly great and the PB you chase may move from 1 week to 12 months if serious injury is obtained.
Your body is extracting energy fighting the illness incurred and resting your body enables it to focus the energy expenditure on getting you well. If you are not fueling your body through lack of appetite and then expect it to perform for you in the gym your illness in all likelihood will stick around longer therefore inhibit you from progressing for an even longer period of time. Rest and Recover and then return to gym when you feel 100%.
Don't be that guy that people hate because you got them sick... Your body is relying on you to help it recover from the illness and resting it allows your immune system full access to recovery. So to the question of Should you train when your sick, the answer is simply give training the cold shoulder!