The immune system is the body's defense against infectious organisms and other invaders. Through a series of steps called the immune response, the immune system attacks organisms and substances that invade body systems and cause disease.
When the humidity is high it feels oppressive outside because sweat doesn't evaporate and provide cooling. When the humidity is low you feel cooler, but your skin dries out and you get dehydrated more easily because more moisture is being evaporated from your body. The same thing applies to crops in high and low humidity. A high water vapor content is necessary to produce clouds and precipitation.
Evaporation (or lack thereof): High humidity makes evaporation, the body's best cooling mechanism, less efficient, so sweat drips off the body instead of evaporating and cooling it. What happens when you overheat. Brain: Dizziness, headache, loss of consciousness, inadequate blood flow to the brain. What to do: Lie down in some shade or air conditioning, wet the body and have a sports drink.