A tailwind will boost up the speed of the runner while a headwind will slow the runner down.
Step-by-step explanation:
A tailwind is a wind that is blowing along the direction of the runner approaching. Whereas a headwind is the wind that blows opposite to the runners approach.
This happens because the body is actually running flat to the wind which gives a resistance to the wind flow. So the wind actually pushes the body forward or backward. If the wind is coming from back, the wind pushes the athlete towards front, increasing his speed by a few miles per hour. When the wind is coming from front, the wind is actually pushing the athlete backwards, which decreases the speed.
The wind also makes it difficult for the athlete to breathe, thereby decreasing the speed to a certain extent in general.