There is no question that the most popular time of year to visit the Valley is in winter. This is partly because the rest of the country is experiencing it's worst weather of the year and in contrast, Phoenix seems like summer. So accept that any time after September and before May is probably best.
RSV: RSV in clinical medicine is the abbreviation for respiratory syncytial virus. See: Respiratory syncytial virus.
RSV Symptoms, Causes & Risk Factors. RSV causes a wide variety of symptoms, ranging from very mild to life threatening. The type and severity of symptoms depend on multiple factors, including the particular strain of the virus and whether the patient has other underlying medical problems.