We study how seasonal climate affects influenza–pneumonia (I-P) mortality using monthly health and climate data over the past 20 years, reduced to mean annual cycle and statistically correlated. Results show that I-P deaths are inversely related to temperature, humidity, and net solar radiation in the United States, South Africa, and Puerto Rico (r < −0.93) via transmission and immune system response. The I-P mortality is 3–10 times as high in winter as in summer, with sharp transitions in autumn and spring. Public health management can rely on seasonal climate-induced fluctuations of I-P mortality to promote healthy lifestyle choices and guide efforts to mitigate epidemic impacts.