Your saliva poision example is actually an example of an implicit contract (for a lack of a better word). That is you assume by default that if someone offered you a drink he doesn’t know about a deadly disease that he may have. If he is sick and does offer you a drink, he breaks this contract. I don’t think this consitutes a muder, but it is a crime.
However there is no implicit or any contract between you and a person just walking next to you. At least I think so. So I don’t think you break any contract if you happen to be sick and someone catches it.