Hi. Welcome to general health community forum.
Weakness and fatiguability is a common phenomena following a viral illness. It occurs due to reduce nutrition and hydration during the illness.
The best way to get over it, is by rest, good nutrition containing plenty of vegetables and fruits and increased fluid consumption. Multivitamin
pills can provide additional benefits and hence can be used.
You wanted to know about the fastest way to become normal following an illness. There you go, this is the fastest method.
Hope this helps. Wish you speedy recovery..