@ysmina-- At the shelter I volunteer at, we give all the cats lysine supplements in their food. It will definitely help a malnourished cat improve her immune system. It's also very helpful for cats that get allergies and upper respiratory infections.
We have lysine in bulk, powder for and we just add some to their food and mix it up. You can definitely get the lysine made for humans, just make sure to lower the dose a bit. I think 500mg a day is a good dose during infections. You should confirm this with your vet though.