"The doctors office couldn't get me in because they were flooded with people calling" about kids with similar symptoms," St. John said.
A positive test in the ER confirmed RSV. The infant developed a rapid heart rate and had to be hospitalised overnight. His 16-month-old sister, Lulabelle, also contracted the virus but was not as sick and didn't need hospitalisation.
St. John said she wondered if it might be Covid-19 because it's the wrong season for RSV.
"I can't say I was relieved, because I know RSV is just as bad," she said.
Children infected with either virus usually develop only mild illness but for some, these infections can be serious.
Among US kids under age 5, RSV typically leads to 2 million doctor-office visits each year, 58,000 hospitalisations and up to 500 deaths — higher than the estimated toll on kids from Covid-19.
Among adults aged 65 and up, RSV can lead to pneumonia and causes almost 180,000 hospitalisation's and 14,000 deaths yearly. Cases in kids and adults usually occur in fall through early spring.
Off-season cases in Australia were a tip-off that the same might happen in the United States, said Dr. Larry Kociolek, an infectious disease specialist with Chicago's Lurie Children's Hospital.
Typically, infants are exposed to RSV during the first year of life, often when older siblings become infected in school and bring the virus home, Kociolek said. But, he added, ''there were a lot of kids and babies who were not exposed to RSV in winter of 2020 and winter of 2021. That just leaves a much larger proportion of susceptible infants".
In infants, symptoms may include fussiness, poor feeding, fever and lethargy. Children may have runny noses, decreased appetite, coughs and wheezing.
But in very young infants and those born prematurely, the virus can cause small airways in the lungs to become swollen and filled with mucous. Babies who develop this condition, called bronchiolitis, may require hospitalisation and oxygen or ventilator treatment.
There is no approved treatment for RSV, although a once-monthly injection of an antibody-based medicine is sometimes prescribed before and throughout RSV season to help prevent severe RSV lung problems in premature infants and other babies at risk for serious disease.
Reinfections are common but typically cause milder symptoms than the initial illness.
Kociolek said the recent unusual surge in cases could be partly due to more testing because of Covid-19 fears. In usual times, parents may dismiss RSV symptoms as nothing serious but now may fear they signal the pandemic virus.
RSV spreads through contact with airborne droplets from an infected person, but it's much more likely than Covid-19 to linger on skin and other surfaces including toys, which can also be a source of transmission.
RSV is among reasons why daycare centres and preschools often have strict policies about keeping kids with coughs home from school.
"A lot of parents think, 'Oh well, it's just a cold, they're fine to go to school," said Diana Blackwell, director of children's programmes at Auburn University-Harris Early Learning Center in Birmingham, Alabama.
Despite strict cleaning measures, several children at her centre have become sick with RSV in recent weeks, including her own 4-month-old son. He had violent coughing spells and was prescribed medicine often used to treat breathing problems in asthma, but did not need to be hospitalised.
She called the summertime outbreak at her centre "just weird."
"It didn't even cross my mind that it would end up being something like RSV," Blackwell said.
Dr Mary Caserta, a member of the American Academy of Pediatrics' infectious diseases committee and a professor at the University of Rochester, said parents should be aware of the unusually timed virus activity and seek medical care if babies appear very ill or have trouble breathing.
RSV is one reason why paediatricians often caution parents of young babies to avoid crowds in winter cough and cold season.
"Covid has made people so hungry to be with other people that it would be hard now" to make the same recommendation, Caserta said.
Whether the unusual summertime virus activity foreshadows less-than-usual RSV this coming winter is uncertain, she said.
"I've given up in any way trying to forecast the future."
