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.
Croup and RSV (Respiratory Syncytial Virus) are common childhood viral illnesses that predominate during late fall, winter, and early spring. Croup, by definition, is an upper respiratory tract infection that results in inflammation and increased mucus production in the larynx, trachea (windpipe), and bronchial tubes.