My sister has recently caught mumps so I have done some research. Being 70 odd years does not build up a natural immuity im afraid. To be immune you must have experienced this virus before i.E you must have had mumps before or had the vaccine.

The mumps virus is spread through infected saliva. If you had a limited contact with this person it is unlikely you will get it ( just my opinion) it is spread through sneezing and coughing.

I hope this helps, you may also like to know that if you have been infected you will not get ill for about two weeks, if all goes well and you are fine, get an mmr vaccine done.