Do you understand how Zika is transmitted? You can't catch Zika from another person. The virus is spread by mosquitoes, they are the vector. A mosquito has to bite someone who is infected with Zika, and then bite someone else. And there is a specific type of mosquito that carries it, so unless you have those types of mosquitoes in your area, if a guest stayed with you and was suffering from Zika, you couldn't get the virus from them.
I realize you were talking about illnesses in general, but there is a lot of misunderstanding about what diseases are contagious from others and how.
And every time you go out in public, you are exposed to all manner of bacteria and viruses.