kitten has runny poop

I rescued two kittens six weeks ago. I started them on kitten milk right away. They
Eventually went to kibble. They started off with just loose poops but now its completely waterery. They are losing weight. Now my two yr old cat has it. What could this be since it was passed along?

It's possible it's parasites and/or worms. I'd take some of the poop to the vet for testing.  You also need to get them some medication to stop the diarrhea.

u need to stop the diarrhea quick , watery diarrhea can dehydrate kittens in very short time and can be fatal. there could be a few possibility, parasite (eg, giardia, coccidosis), bacterial (eg,, e coli), fungal(mole). u better check and see if the kitten milk u feeding is good, sometimes even if it's not expired there's still a good chance it whent spoiled due to heat during transportation or improper storage. Better taste the kitten milk by urself, see if there's a rich milk and sweet taste, there shouldn't be any bitter taste in the kitten milk and u should stop feeding even if there's any slight weird taste from the milk. in the mean time u should get them to vet for sub q fluids to deal with dehydration, vitamin b 12, vitmin c, 2 antibiotics (fenbendazole, sulfamethoxazole/trimethoprim) for  getting rid icsof any parasites and help to stop their diarrhea. Ask the vet to inspect their feces and try to idenfity the cause with a microscope. also u gonna need probiotics once u finish feeding them antibiotics to restore the balance of their intestines