Most doctors had predicted the fever would end when the cold weather returned. Almost all epidemics followed the same pattern, striking during warm weather, disappearing with the first hard frost. Daniel Defoe, in recording the end of the Black Death that visited London in 1665, wrote that "the winter weather came on apace [and] most of those that had fallen sick recovered." —An American Plague,
Jim Murphy

Which two cause-and-effect relationships are based on facts and could be proven?

Yellow fever would end with cold weather.
Diseases always spread faster in the summer.
Yellow fever would continue throughout the winter months.
The Black Death decreased during the winter.
People are always healthier in the winter.