Illnesses include croup, bronchiolitis and pneumonitis. Paramyxoviruses replicate in the cytoplasm and are released by budding. Virus-specific antigens include those associated with the envelope and those contained within the nucleocapsid. The host range for the Paramyxoviruses includes humans and monkeys. Parainfluenza viruses are the most common cause of croup and there are 4 serotypes, numbered 1-4.
Some severe manifestations can result from infection but there is only one serotype.
Another acute childhood disease, measles virus commonly causes a fever with a rash, occasionally producing conjunctivitis and, sometimes, pneumonia. Only one serotype exists and disease is limited to humans and monkeys. Respiratory Syncytial virus is a major cause of bronchiolitis and pneumonia in infants under 1 year. Reinfection in adults usually involves the upper respiratory tract. The viruses produce a characteristic syncytia formation, hence the name.