If you fall ill during your trek, the course of action depends on what's wrong. If it's altitude sickness, moving to lower altitudes and taking altitude medication will solve most cases; if it's pretty severe, your guide will arrange for an emergency evacuation by helicopter (be sure that emergency helicopter rescue is covered in your travel insurance). If you have a stomach sickness or other food or water-related illnesses, your guide will have medication on-hand for most common illnesses.