My in-laws have a home helper who put covered non-kosher food in their microwave once. My wife then kashered the microwave but I am concerned that the helper might do this regularly when we are not there and my mother-in-law may forget to kasher the microwave and heat some kosher food on a kosher plate in the microwave without kashering it after the helper puts covered non-kosher food in the microwave. If that happens would that make the kosher plate that was put in the microwave treif along with other kosher cutlery etc... As the non-kosher food was covered, won't that cause no spillage of the treif food in the microwave?

Thank you for your question, If the food was completely covered (preferably with two coverings, such as a bag over a food box) the microwave stays kosher (even though this situation is not a priori proper). Therefore, if you use the microwave I think it would be a good idea to do what your wife did; namely, kasher the microwave before you use it. Koshering the microwave only takes a few minutes. So it is not too burdensome. In any event, it wouldn't treif up your dishes if the nonkosher food was always hermetically covered. All the best!