The information provided here is not a replacement for a doctor. You shouldn't use it for the purpose of self-diagnosing or self-medicating but rather so you can have a more informed discussion with a professional TCM practitioner.
In Chinese Medicine weakness after illness can be treated by one herbs that we have on record. Here is the list below.
Because it is a herb specifically indicated to treat weakness after illness as can be seen on Crown Prince Ginseng's page.
Its main actions are: Tonifies the the Lung and Spleen Qi. Supplies Body Fluids.