Why is my cat is hissing at my other cat?

I have 2 cats. Both brothers at about 6 months old (kittens) I woke up this morning and found one of them has been sick and has had the "runs" everywhere, especially down his tail and legs. So I washed his lower end in the sink and he s now all wet. My other cat won t stop hissing at him. Anyone know why?
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