After camping, 14 of a group of 20 became ill within a couple of days. Symptoms including a high fever, sore throat, vomiting or lethargy lasted for a few days. The lake water was clean. Could these be signs of food or toxic poisoning?
The source of this kind of mass outbreak of illness can be difficult to pinpoint, unless you bring in an epidemiologist (a doctor or scientist who studies the spread of disease). It is unlikely, though not impossible, that the symptoms you describe are signs of inhaling toxic fumes. Such effects would usually show up immediately.
When you mention the lake water being clean, I assume you mean for swimming or fishing. Despite the assurance that the water is clean, unpurified lake water may contain pathogens that can cause illness and should not be used for drinking, washing dishes or brushing teeth.
Your drinking water could be the culprit here, but the most common waterborne illnesses usually have diarrhea as a symptom, which your campers did not. You are right to consider foodborne illness as a culprit.
Proper food-handling techniques are not always easy to maintain when you are out in the woods. It is possible that people ate undercooked meat, fish or poultry or food that was left sitting out for too long at the wrong temperature. Even if the food did not make people sick, it is a good idea to review outdoor food safety guidelines before your next camping trip or picnic.
There is another possibility that has nothing to do with the food or water. Those 14 people could have caught the flu or other viral illness. The symptoms and their duration match those of a flu-like bug. A small group of people living in close contact for a period of days creates a favorable environment for a mini-epidemic.