Infections of the lungs and respiratory tract are caused by a respiratory syncytial virus (RSV). The virus is so common that most children have been infected by age 2. Adults can also be infected with respiratory syncytial virus (sin-SISH-ul). Symptoms of RSV are mild in adults and older, healthy children. To relieve discomfort, self-care measures are usually sufficient. It is important to note that RSV can cause severe infection in some people, including babies 12 months and younger (infants), especially premature infants, people with heart or lung disease, older adults, or those with weak immune systems (immunocompromised). In most cases, RSV causes mild, cold-like symptoms. RSV usually clears up in a week or two, but infants and older adults are more likely to experience serious symptoms. It is one of the most common causes of bronchiolitis (inflammation of the small airways of the lungs) and pneumonia (infection of the lungs) in children under one year of age. In some cases when people with chronic obstructive pulmonary disease, asthma, or congestive heart failure suffer from RSV, their symptoms can worsen.