Respiratory syncytial virus, commonly called RSV, is a highly contagious virus that infects the lungs and airways. Nearly every child catches it by age two, and most healthy adults who get it experience nothing worse than a cold. But for infants, older adults, and people with weakened immune systems, RSV can become serious enough to require hospitalization. In the United States alone, RSV is associated with 190,000 to 350,000 hospitalizations and 10,000 to 23,000 deaths each year.
How RSV Infects the Airways
RSV targets the cells lining your respiratory tract. The virus attaches to proteins on the surface of these cells, triggering changes that essentially trick the cell into pulling the virus inside. Once in, RSV hijacks the cell’s machinery to make copies of itself. It also causes neighboring infected cells to fuse together into large clumps called syncytia (the “syncytial” in the virus’s name). This fusion damages the airway lining and triggers inflammation, which narrows the small breathing passages and produces mucus.
In adults with full-sized airways, that swelling is usually manageable. In infants, whose airways are tiny to begin with, even modest inflammation can make breathing difficult. That size difference, combined with an immune system still learning to fight infections, is the main reason babies get hit so much harder.
How RSV Spreads
RSV travels through respiratory droplets when an infected person coughs or sneezes. You can also pick it up through direct contact, like kissing a child who has it, or by touching a contaminated surface and then touching your face. The virus can survive for many hours on hard surfaces like tables, countertops, and crib rails. It lasts shorter periods on soft surfaces like tissues and hands, but long enough to spread easily in daycare centers, schools, and households.
In most of the United States, RSV circulates seasonally from fall through early spring, with activity typically peaking in winter. This pattern overlaps with flu season, which is why respiratory illnesses seem to pile up during colder months.
Symptoms in Babies and Young Children
In infants, the first signs are usually a runny nose and decreased appetite. A cough develops one to three days later, followed by sneezing, fever, and wheezing. Very young infants may not show the classic cold symptoms at all. Instead, they become irritable, less active, and may have pauses in their breathing (apnea).
Most children recover on their own within one to two weeks. But RSV is the leading cause of bronchiolitis (inflammation of the smallest airways in the lungs) and pneumonia in children under one year old. The hospitalization rate for infants under 12 months is strikingly high compared to every other age group. During the 2024-2025 season, roughly 1,117 out of every 100,000 infants in that age range were hospitalized for RSV, and children aged 12 to 23 months weren’t far behind.
Symptoms in Adults
Most adults who catch RSV experience it as a straightforward upper respiratory infection: runny nose, sore throat, cough, headache, fatigue, and mild fever. The illness typically clears up in one to two weeks without any specific treatment.
The risk rises significantly for adults 65 and older, especially those with chronic lung disease, heart failure, or weakened immune systems. RSV can worsen existing conditions like COPD and asthma, and it can progress to pneumonia. Among recorded RSV-associated deaths, 82% occur in adults 65 and older, making this age group the most vulnerable after very young infants. Adults 75 and older are hospitalized at a rate of roughly 427 per 100,000.
Warning Signs That Need Urgent Attention
Certain symptoms signal that RSV has moved beyond a manageable illness. In children, watch for skin pulling inward between the ribs with each breath (called retractions), nostrils flaring during breathing, and short, shallow, or unusually fast breaths. Pauses in breathing are particularly concerning in very young infants. Bluish or grayish color on the skin, lips, or fingernails indicates low oxygen and requires immediate care.
In both children and adults, difficulty breathing, wheezing that gets progressively worse, and signs of dehydration (not drinking enough fluids, fewer wet diapers in babies) all warrant a call to a healthcare professional or a trip to the emergency room.
Treatment and Recovery
There is no antiviral medication that cures RSV. Treatment focuses on managing symptoms while the body fights off the infection. For most people, that means rest, fluids, and over-the-counter fever reducers. Saline nose drops and gentle suctioning can help infants who are too congested to feed comfortably.
Children and adults who develop more severe illness may need hospital care, which typically involves supplemental oxygen and IV fluids to prevent dehydration. In the most serious cases, particularly in very young infants or elderly adults with underlying conditions, mechanical breathing support may be necessary. Hospital stays for RSV bronchiolitis in infants generally last a few days, though premature babies and those with heart or lung conditions may need longer care.
Vaccines for Adults
Three RSV vaccines are now available for adults aged 50 and older. The CDC recommends a single dose for all adults 75 and older, and for adults 50 to 74 who are at increased risk for severe RSV illness. Two of the vaccines are also approved for higher-risk adults as young as 18.
Real-world data from the 2023-2024 RSV season showed strong protection. GSK’s Arexvy was approximately 77% effective at preventing RSV-related emergency department visits and 83% effective at preventing hospitalizations in adults 60 and older. Pfizer’s Abrysvo showed similar performance, at roughly 79% and 73% effective for those same outcomes. Moderna’s mResvia demonstrated about 80% efficacy against symptomatic RSV in the first four months after vaccination, though that protection gradually declined to around 34% by 18 months.
Protecting Infants
Babies are too young for traditional vaccines, but a preventive antibody called nirsevimab (brand name Beyfortus) gives them borrowed protection. It’s a single injection that provides antibodies to fight RSV, rather than training the immune system to make its own. The CDC recommends it for all infants during RSV season, which runs October through March in most of the U.S.
For babies born during RSV season (October through March), the injection should ideally be given within the first week of life, during the birth hospitalization. Babies born outside that window (April through September) should receive it just before the season starts, typically in October or November. Dosing is based on weight and age: infants under 8 months get either 50 mg or 100 mg depending on whether they weigh less or more than about 11 pounds. Older infants aged 8 to 19 months who are at increased risk for severe disease and entering their second RSV season receive a larger dose of 200 mg, given as two separate injections.
How RSV Is Diagnosed
When testing is needed, two main options exist. PCR-based tests (which detect the virus’s genetic material) are highly sensitive and considered the gold standard. Rapid antigen tests are less sensitive but return results faster, making them useful in urgent care and emergency settings. In many mild cases, especially in otherwise healthy adults, testing isn’t necessary at all since the treatment is the same regardless of whether the respiratory infection is RSV, a common cold, or another virus.