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Very gently with plenty of rest maybe ?
And try it outside in the fresh air - nature is much nicer than a gym.
A lot will depend on how badly you were hit by it. People can be three days in hospital with it, or three months. Progress is encouraging or otherwise depending more on expectations rather than on any actual level of achievement. Keep expectations low, and above all, no comparisons with your former self.
The other thought which may or may not apply, is that sometimes an increase in training is possible only if matched by an equal increase in time resting - feet up, dozing over book or telly. The time with feet up is probably a wise investment, and activity matched by extra rest may be the quickest way back to a satisfying level of activity.
I take it that the illness is well and truly gone. Viruses are like scorpions - the sting is often in the tail. If any trace elements remain of active viral illness, don't risk doing anything. Depressing to do nothing, on top of the illness which is depressing in and of itself, but unfortunately wise. Sit tight.