The effects of seasonal variability on developmental rates and life cycles of wildlife populations have a dramatic impact on disease transmission and the timing of disease outbreaks.Through deterministic and stochastic mathematical models and numerical simulations we study effects of seasonal variations on infectious diseases. In particular, we examine how the seasonal variations affect the timing of disease outbreaks in populations and in human spillover in three zoonotic diseases, avian influenza, dengue, and Lyme disease. The seasonal effects of wild duck migration and environmental transmission of avian influenza virus are examined in an avian influenza model. The seasonal variability of the vector populations, mosquitos or ticks, on disease outbreaks is investigated in models for dengue and Lyme disease. Applying a multitype branching process approximation near the disease-free solution, we compute the probability of a disease outbreak. We find that the seasonal periodicities in the models impact the probability of a disease outbreak, which is also periodic and depends on the environment, life cycles of vectors, and host densities. The methods developed here can be helpful to public health professionals when assessing intervention and control measures in seasonally-driven infectious diseases. This is ongoing joint work with faculty, undergraduate students, and graduate students of TTU and TTU/NSF sponsored programs.