Hi there,
a classmate did a 10-week externship at Plum Island and was disappointed because (naturally) they didn't let him DO anything (he wanted a research project, all they let him do was tag along)> Understandable, as they've got some pretty scary germs there... I would suggest the CDC's program... you have to apply for it pretty far in advance. And maybe you have to be a US citizen? I'm also interested in public health (but working in private practice for now). What are your special interests?
And I have to recommend that you look into the MPVM program at Davis. With your background and experience as an international student I think you would fit in well. Friends of mine are in the program now and they enjoy it a lot, and there are lots of international students in the program.
Best of luck!