Mycoplasma gallisepticum causes disease in poultry and emerged as a novel pathogen in house finches resulting in a mycoplasmal conjunctivitis epidemic across North America after a single successful host jump from poultry to house finches. The rapid spread of the epidemic across eastern North America, causing a decline in host abundance, has been documented. Once established, disease prevalence showed regular seasonal variation: a late summer/early autumn peak, a mid-December minimum, followed by a late winter peak and a breeding season minimum, which requires seasonal reproduction (providing an autumn pulse of naïve hosts) and winter social aggregation as well as partial immunity of recovered birds. Virulence evolved rapidly: in eastern populations it increased once the disease had become endemic. In western North America the established strain was of much lower virulence, but once established also increased in virulence. We show that virulence may evolve in opposite directions depending on selection pressures. A detailed study showed that disease decreased survival and mobility and a high proportion of birds recovered; also that re-observation rates of clinically diseased birds are often different from those of asymptomatic birds; only calculations which include this effect allow an accurate estimate of disease prevalence.