Why is my kitten vomiting and having diarrhoea?

I have a 8 month old kitten, who is eatin regularly, spends 80% outdoors, yesterday he had diarreoha which had blood in it, today he also had diarreoha but no blood. Today he was sick. What can this be? I have another kitten but he has not got diarreoha. Both kittens have been vacinated and de-wormed, and there food has not been changed?