Hair loss due to Disease like typhoid

The type of hair loss you experienced is called Telogen Effluvium and is due to the shock to the hair follicle from a severe illness or injury (physical or mental). Normally when the body recovers the hair grows back so within a year from when you experience hair loss you should have regained your normal hair.

Since this didn't happen it is possible that you also suffer from Androgenic Alopecia so you need to see a dermatologist to confirm or rule that out.

Revivogen and no other product work for Telogen Effluvium which doesn't need a treatment anyway as the lost hairs grow back within a year. However if you have Androgenic Alopecia Revivogen can work to stop further hair loss and to regrow the hairs you lost during the past three years. This is because after 3 years the hair follicles die and no product can revive them. So although you can't regrow the hair you lost 4 years ago, if you have hereditary hair loss (androgenic Alopecia) you can still stop its progression and even grow some of the hairs you lost during the past three years.