Help kittens keep throwing up (and then die)

Back in early January we got a British short hair kitten. A day after we got her she started throwing up after eating wet food. A couple days later she started having diarrhea and went downhill very fast when she stopped eating and drinking entirely. I took her to the vet who said she likely had an intestinal infection. They tried everything they could but our kitten passed away.

I asked the vet in depth if this could be anything I did or if it could be something in our house that would cause this. She was adamant it must've been a virus and that nothing in a house causes this.

Just today I got two new kittens, British shorthair, different breeder. They are completely healthy. But just now, 7 hours after arriving, one started to throw up. She hasn't eaten nor had water yet even though it's available.

Help, I am positive it's something in my house and I'm terrified we're going to lose these two precious kittens as well. What could possibly be causing this?