Weather changes. The human body responses to changes in barometric pressure - this explains why people with joint conditions can "predict" rain - as the barometric pressure drops in front of a storm - our joints and the fluid in them are subject to minute pressure changes. Also the humidity of the air can cause our noses to produce more mucus in an attempt to humidify the air we breath.
POSSIBLY SOME. Certain weather changes can bring in different allergens which can cause people to have flares in asthma and allergies. Pressure changes that can occur with weather changes seem to correspond to an increase in pain in some arthritic canges. Increased heat and humidity can make patients with COPD more short of breath. These are a few examples where weather can play a role.