Different patterns of influenza incidence in a total population, caused by different patterns of social interaction among four etiologically distinct age groups: children <5 years (black lines), children 6 to 20 years (red lines), adults (green lines), and the elderly (blue lines). All 12 modeled scenarios used the same population size and demography. The only differences among the modeled scenarios that yielded constant incidence of influenza (A), rapidly stabilizing, oscillating incidence of influenza (B), or long-term, periodic, oscillating incidence of influenza (C) were in the social interaction rates among these etiological groups. All parameter values and interaction rates held constant throughout each scenario.