<unk>  infection can present with a wide variety of signs and symptoms that range from mild upper<unk>  tract infections (URTI) to severe and potentially life-threatening lower<unk>  tract infections (LRTI) requiring hospitalization and mechanical ventilation. While<unk>  can cause<unk>  tract infections in people of all ages and is among the most common childhood infections, its presentation often varies between age groups and immune status. Reinfection is common throughout life, but infants and the elderly remain at highest risk for symptomatic infection. Children Most children will experience at least one<unk>  infection by age 2. Most childhood<unk>  infections are fairly self-limited with typical upper<unk>  tract signs and symptoms, such as nasal congestion, runny nose, cough, and low-grade fever. Inflammation of the nasal mucosa (rhinitis) and throat (pharyngitis), as well as redness of the eyes (conjunctival infection), may be seen on exam. Approximately 15–50% of children will go on to develop more serious lower<unk>  tracts infections, such as bronchiolitis, viral pneumonia, or croup. Infants are at the highest risk of disease progression. Video:<unk>  and Bronchiolitis Bronchiolitis is a common lower<unk>  tract infection characterized by inflammation and obstruction of the small airways in the lungs. While several viruses can cause bronchiolitis,<unk>  is responsible for about 70% of cases. It usually presents with 2 to 4 days of runny nose and congestion followed by worsening cough, noisy breathing, tachypnea (fast breathing), and wheezing. As infants work harder to breathe, they can also show signs of<unk>  distress, such as subcostal retractions (when the belly pulls under the ribcage), intercostal retractions (when the muscles between the ribs pull inward), grunting, and nasal flaring. If the child has not been able to feed adequately, signs of dehydration may also be present. Fever may be present, but high-grade fever is uncommon. Crackles and wheezing can often be heard on auscultation, and oxygen saturation levels may be decreased. In very young infants under 6 weeks of age, and particularly in premature infants, signs of infection may be less specific. They may have minimal<unk>  involvement. Instead, they may exhibit decreased activity, irritability, poor feeding, or breathing with difficulties. This can also be accompanied by apneic spells, or brief pauses in breathing. Adults Reinfection with<unk>  remains common throughout life. Reinfection in adulthood often produces only mild to moderate symptoms indistinguishable from the common cold or sinus infection. Infection may also be asymptomatic. If present, symptoms are generally isolated to the upper<unk>  tract: runny nose, sore throat, fever, and malaise. In the vast majority of cases, nasal congestion precedes the development of cough. In contrast to other upper<unk>  infections,<unk>  is also more likely to cause new onset wheeze in adults. Only about 25% of infected adults will progress to significant lower<unk>  tract infection, such as bronchitis or tracheobronchitis. While<unk>  very rarely causes severe disease in healthy adults, it can cause significant morbidity and mortality in the elderly and in those with underlying immune compromise or cardiopulmonary disease. Older adults have a similar presentation to younger adults but tend to have greater symptom severity with increased risk of lower<unk>  tract involvement. In particular, the elderly are more likely to experience pneumonia,<unk>  distress, and death. Immunocompromised In both adults and children, those who are immunocompromised are at an increased risk of severe infection with<unk> . Infected individuals in this group are more likely to progress from upper to lower<unk>  tract involvement and have prolonged viral shedding. Symptom severity seems to be closely related to the extent of immune suppression. Those who have undergone hematopoietic stem cell transplant (HSCT), intensive chemotherapy, and lung transplant are particularly susceptible. Bone marrow transplant patients appear to be at highest risk, especially prior to marrow engraftment. In this group,<unk>  infection carries a nearly 80% risk of both pneumonia and death. Complications Population Complications of<unk>  infection Children Short term, hospitalized children are at risk of developing: * Atelectasis / pulmonary infiltrates * Acute otitis media * Lung hyperinflation *<unk>  failure * Apnea * Bacterial pneumonia Long term, children are at risk of developing the following chronic conditions that may persist into adulthood: * Asthma & recurrent wheezing, particularly among those with severe<unk>  infection in early life * Allergy Adults The following are more common in elderly adults or those with underlying immunocompromise or cardiopulmonary conditions: * Pneumonia *<unk>  distress * Acute exacerbation of underlying chronic illness (COPD, asthma, congestive heart failure) * Permanent decline in lung function in patients with COPD Immunocompromised Some immunocompromised groups are at higher risk of specific complications, such as: * Bone marrow transplant recipients → pneumonia, death * Lung transplant recipients → chronic graft dysfunction, acute pneumonitis, obliterative bronchiolitis
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