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.
rabies virus an RNA virus of the rhabdovirus group that causes rabies. respiratory syncytial virus (RSV) any of a genus of single-stranded paramyxoviruses; the name is derived from the type of disease produced (respiratory infection) and the microscopic appearance of the viruses in cell cultures.