Dehydration

The asteroid hits the east coast of America at 9:30 p.m in the year 2026. It doesn't make headlines or news but it is the end of civilization. A parasitic pathogen escapes the charred remains and quickly spreads through water. The infected become husks as water is sucked from there body and they become controlled by the pathogen.
Alex Vence is saved from the carnage as his plane experiences technical difficulties due to the asteroids. They make an emergency landing in the middle of the Atlantic Ocean. It takes them a week to paddle back to mainland and by then the plague is rampant. They are forced to survive.