This is, of course, a matter of opinion. My opinion is that HIV AIDS can be a valid foreign policy concern. The reason for this is that the US has an interest in creating and supporting stable countries, in Africa and other places, that will be able to develop economically and politically. HIV AIDS makes it more difficult for this to happen.
A world in which more countries are stable and are developing politically and economically is a safer and more prosperous world. If less developed countries become more developed, their people are less likely to get involved in wars and terrorism, for example. If their political systems develop, they will be less likely to experience coups and other such disruptions.
HIV AIDS makes development harder because it takes away from the pool of workers and because it weakens their social structures. It is in the US's interests to prevent this from happening. Therefore, HIV AIDS is a valid foreign policy concern.