Can someone explain for me Feline viruses?

If the cat has a virus this means the virus won't go right? He will be sick for the rest of his life?

If this is true why vets prescribe antiviral medications? What's the point?

My cat is on his second day with Lysine hope this will work 

And does this means I can't foster kittens anymore?  

No, he won't be sick the rest of his life, but could have whats called outbreaks. The healthier a cat is, and the more stress free he is, the less likely outbreaks will occur. A good immune system keeps the virus in check. Treatments revolve around relieving the symptoms until the immune system can fight it off. Antivirals are used to hopefully shorten the length of the outbreak and to relieve symptoms.

The L-lysine you already have your cat on will also help keep outbreaks at bay, so you will want to continue that supplement.

I believe that if your cat is not having an active out break, it would be ok to still foster. But I'd ask your vet about that.