The answer to this question depends on the nature of your illness. Is it a cold, the flu or an allergy? Most people confuse a cold with the flu, which are caused by different viruses. It is true that the symptoms of these two illnesses are similar. With a cold, you have a runny nose, feel tired and weak. Muscle aches often appear but they are not very intense. You may also have a fever and headaches, but the symptoms are not very marked.
The flu, on the other hand, is much more aggressive and its symptoms appear suddenly: high fever, great fatigue, aches and pains… You are not well at all and you end up confined to bed for several days.
It is important to know that the immune system is strongly taxed in case of flu. Training in these conditions will not only be detrimental to muscle growth but also to your health.
Source : Paul