I'm worried about my kitten..

Hello everyone,

We found a kitten in the bushes Monday, without his mom/family. He also have an attack of conjonctivitis. So we kept him. First day, he was scared, but ate cat food and drank milk. Then yesterday, he started to get less and less active and sits most of the time. And today, I feel like he's very weak, you can grab him and he says nothing. He only sleeps and don't eat and drink. I've searched for a solution on Internet and apparently he's sad of losing his brotherhood/family, but it don't convince me..

Can you guys know what is happening to him?

EDIT: He had Tifus. Thanks for the teplies everyone.