Humans can't swallow and breathe at the same time because a small flap of cartilage called the epiglottis is on duty in your throat, directing traffic. Normally, air travels easily through the trachea and into the lungs, but when you're swallowing food or liquid, the epiglottis, a leaf-shaped lid for the larynx, moves into place to block the airway and direct the contents to the esophagus, on the way to the stomach.
When the food passes this checkpoint -- at the entrance to the larynx, also called the voice box -- extrinsic muscles relax, the epiglottis retracts upward and breathing resumes normally.
This process is unique to humans, as other mammals can breathe and swallow at the same time. The reason? The voice box is positioned lower in the human throat.
More about the epiglottis:
After coughing seriously, is it possible that some food particles will still remain in the lungs if they miss their way to the stomach? What danger will this pose? --NKD