Why do cats recover from distemper after surviving 7 days?

The saying that cat distemper improves after 7 days is because the virus’s development cycle is about 7 days; surviving past day 7 means passing the danger period and generally recovering gradually. Days 3-5 of infection are the most dangerous – on day 3 white-cell counts drop, on day 5 they rise again. After effective treatment, symptoms usually ease around day 7: vomiting frequency drops and appetite and spirits improve.