Where's the hard evidence that colder weather is better for human health?

Hospitals witness five-week rise in flu patients: CDC - Taipei Times

Cold fronts and wet weather in recent weeks have contributed to the increasing number of people affected by the flu, the CDC said.
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Not only has the number of outpatient and emergency visits resulting from flu and flu-like symptoms been on the rise for five straight weeks, hospitals across the country have seen huge increases in the number of emergency visits, CDC Deputy Director-General Lin Ting (林頂) said.
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On the other hand, the cold weather has helped conditions dramatically improve in terms of dengue fever, especially in southern Taiwan where only 16 cases of indigenous dengue fever were confirmed last week, the CDC said.