In what way weather effects virus spread is beyond my understanding.
various possible ways: for instance (theoretically) heat and humidity might effect how long virus survives intact outside of the host, or increased UV light exposure could speed up virus breakdown.
UV light makes sense, humidity and heat does not, because if I am not mistaken; in summer humidity can be substantially higher then in winter and body temperature is fairly high compered to outside average.
Consider that just raising the internal body temperature by a few degrees is a major mechanic in your immune system. Many diseases are not tuned for 100 degrees Fahrenheit