I am grateful that you explained that to me politely.

Hi Mika I will try to explain. I love cats too. So I was able to guess what you meant by dishes from the context of the rest of your piece of writing. In English we usually just abbreviate "veterinary," to vet. In some places in Northern England, people still say "Veterinary," because it is short for "Veterinary Surgeon," but in most of England it is better to simply say 'vet.' You meant that, because your cat is a pure-breed cat he is more likely to develop an illness or catch a disease than a mixed-breed cat (we often say cross-breed in English). However, your vet is wrong to say that the fact that your cat has been castrated makes him more likely to become ill. The opposite is true. It is, generally, true that mixed breeds of any animals are more robust (in terms of their health) than pure-breeds because mixed-breed animals avoid inheriting lots of "bad," genes from their parents. I changed your last sentence to show that mixed breed cats are less likely to suffer disease than pure-breeds (we use the word "pedigree," in English to refer to pure-breeds). It does not make sense to compare mixed-breed cats with "in-door cats," because some in-door cats are of mixed breed. Indeed with cats, unlike dogs, the vast majority of animals are mixed-breed animals.