Is there a term for something that is tainted with another's saliva?
I do not believe there is a single word that exactly means the same as jutha, as that concept is particular to Hinduism. We would use the word as a loanword if necessary to be specific.
However, there are several general terms for left over food from plates, as opposed to leftover food which was never served (that being called leftovers).
- slops
- swill (specific to food waste given to pigs)
- scraps
- food waste
- remains
As an aside, whilst this concept persists as a religious observance, there is the opposite end of the scale that should be more widely known, see the Filipino slum food pagpag.
I can't think of a specific word, but "I don't want to share your germs" is a typical way to say it. It's a tiny bit offensive for some people if they are told this, but most people understand.
There is a related phrase that is used which might be appropriate: "double dipping." This is where (for example) a person who is taking food from a shared bowl eats using a spoon and then places the same spoon back into the food for another bite (could also be a potato chip, vegetable, etc.) [ see: http://www.youtube.com/watch?v=1J3w4cS2MvM ]
Is there an English word for something that is not eatable or usable by others because it has someone's saliva on it?
Not specifically, but people might say it was unhygenic (to eat someone-else's left-over food).