your theory is flawed. here's a better one:
In 2026, our manned flight to mars returns to Earth with an unusual substance. While on delivery to an enclosed facility, the person delivering the substance drops it upon entering the containment area and infects the three scientists within and him/herself. Without any symptoms showing, the delivery man is fired and finds common work among the populace and unknowingly spreads the plague to everyone he comes into contact and so forth an outbreak occurs.
Later in 2028, the country, and most likely the world is overtaken by the plague and very few people remain, with some carriers and some immune and the world's armed forces diminished to a small number with only a few superiors.
