The human parainfluenza viruses (HPIV1-4) are associated with upper and lower respiratory tract illness, upper respiratory symptoms include fever, runny nose, and cough while lower respiratory illness may include laryngotracheobronchitis (croup), bronchitis, bronchiolitis, and pneumonia. Infections are more severe in children, older adults and those with weakened immune systems. HPIV-1 is the leading cause of croup in children, a respiratory illness that manifests as swelling near the vocal cords and in other parts of the upper respiratory system. HPIV-2 is also associated with croup but is generally less detected. Outbreaks are generally more common in the winter. HPIV-3 infection is commonly associated with pneumonia and bronchiolitis, it often causes infections in the spring and early summer. HPIV-4 has been indicated in a large portion of asymptomatic or mild infections.