Respiratory syncytial virus (RSV), also called human respiratory syncytial virus (hRSV) and human orthopneumovirus, is a very common, contagious virus that causes infections of the respiratory tract. It is a negative-sense, single-stranded RNA virus, and its name is derived from the large syncytia that form when infected cells fuse together.While RSV is the single most common cause of respiratory hospitalization in infants, reinfection remains common throughout the lifetime and it is an important pathogen in all age groups. [Source: Wikipedia 
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