To prevent muscle loss is a totally different question related to gym exercises an dall but in your case 1. Routinely to build up muscle during exercise it to work out properly as per the rules or advice an also protein. Powders or shakes post work out help out under advice 2. In ur case if you have fever sore throat advisable not to work out till resolved and also the fever throat problem will aggravate the muscle loss if at all u r having. Not go into rhabdomyolysis patho muscle loss