Sick people are sort of viscerally gross, we're repulsed by wet coughs, even the sound of vomiting some people can't cope with, not even on a movie -- it's an evolutionary response, I think. Animals probably have similar feelings around sick people.
The emotion of disgust is related to disease and how likely we are to catch something. You will likely find an open festering wound disgusting, rotten food or a filthy bathroom.
Can you explain why you think this is a modern thing?
IMO if someone in your group starts vomiting can mean the food we just ate is bad so would be safe to throw up sooner then later.
I was raised with animals in a village and I have no trouble with cleaning animal excrement but human excrement's smell instantly has an effect on my stomach(I had to get some samples for a medical test from my son poop). But sure there are daily working with such thing and they get used to it.
Why do you think so? People were not shitting in the corner of the room like animals(if they had a choice), they would go shit outside and i am not aware of any historical ritual involving shit but let me know if there is some perspective I am missing.
I think there are still many people around the world that are living in extreme poverty and I am sure they still feel disgusted by shit,bad food and putrefied bodies - so are you basing your theory on something you read or is something you think is correct , I will be happy to change my mind so please inform me.
In an article on Cholera or more it's solution in London building sewers the author reposted a letter to the news paper that castigated the upper classes. The gist of it was 'You say we don't mind living in filth and stink. Assuredly we do not like it at all'
We probably traded off getting some very contagious diseases but sticking together to living off by ourselves. In the long run it seemed to have been a good decision for our species.