Any type of upper respiratory infection or virus will affect an individual’s performance to a different degree, even after recovery. 
According to Sport Scientist, Chérien Roux, "the severity of the infection influences how soon a person can return to participating in sports."

A lot of people are uncertain as to what steps they should follow when resuming training. Below Roux outlines a five step plan, starting with a very basic self-assessment:

Before even considering to start training again, determine what your body would be capable of handling. An easy test is to walk 500 metres on a flat surface. If you feel fatigued during or after the walk, then you are obviously not ready to resume any form of training yet. It is also important to listen to your body. If you do not feel up to training, then do not force yourself to start training.

Here is Roux's Five Stage plan:

Stage 1:

Begin training only once you have been cleared to do so by your doctor and you have successfully completed your self-assessment as described above. With stage 1 you can start walking again and perform daily tasks. This stage should last about 10 days as your body still needs some time to recover following the respiratory illness, as you still need to protect your cardiorespiratory system. Monitor your symptoms during activities to prevent exertion. Your maximum heart rate should remain below 50%.

Stage 2:

During stage 2 one can start performing light activities such as: longer walks, slow- to steady jogging and stationary cycling. Resistance training is not recommended as you do not want your heart rate to exceed 70% of its maximum capacity. The purpose of this stage is to systematically introduce exercise without putting too much strain on the cardiac muscles. Workouts should be no longer than 15 minutes. This stage should last a minimum of two days, to see how you are coping with the light activity and move from there onwards. I suggest that you should continue with stage 2 for at least one week in order to allow your body to fully adapt to exercise again.

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Stage 3A:

During this stage you still continue with light training but you can start increasing your training frequency and you can start doing simple movement activities such as drills. You may increase your heart rate, but it must not exceed 80% of its maximum rate. Workouts should be no longer than 30 minutes. This stage can last a minimum of 1 day, but I would suggest to stay in this stage for at least a week. Stage 3A allows you to adapt to an increase in training frequency.

Stage 3B:

Evolving out of 3A, you can start by increasing the duration of your exercise. Again it is important that your heart rate does not exceed 80% of its maximum rate. Do not train more than 45 minutes per workout. One day is the minimum amount of days recommended during this stage, but again I would suggest a full week. Stage 3B systematically introduces you to exercising for longer durations.

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Stage 4:

During this stage you can start incorporating your normal training activities again. Both the intensity and duration of your exercise can increase. Stage 4 should last for a minimum of two days, but again I would recommend that it lasts for a week. Your heart rate should still not exceed 80% of its maximum rate. The duration of your workouts should be a maximum of 60 minutes. By now more strain can be placed on the cardiorespiratory system. You can also systematically incorporate weight training keeping the above mentioned in mind.

Stage 5:

The final stage allows you to go back to your normal training routine. Systematically resume your normal training schedule back into your routine.

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