Plan ahead for many seasonal diseases that could derail even the best laid clinical trials.

Seasonal variation in the incidence of diseases has been observed for centuries, dating back at least to ancient Greece, and yet scientific understanding of its underlying mechanisms remains relatively rudimentary for many diseases.1 Seasonality is not only an important factor in common infectious diseases—such as influenza, chickenpox, and measles—but also in noninfectious diseases such as seasonal affective disorder (SAD) and rheumatoid arthritis (RA).