Contagion
(2001)

The boy (Matteo Crismani) is immune because he survived an ebola epidemic in Rwanda where his parents died, and has the antibodies to save the president (Bruce Boxleitner) and everybody else as well.

Terrorist Laura Crowley (Lin Shaye) tries to escape and take boy with her, but is pushed from the hospital roof by virologist Dr. Diane Landis (Megan Gallagher) who proceeds to develop a serum that saves everybody.

 Thanks Tilman!