The mother of an 8-year-old boy made the exact right move when she took him to an after-hours clinic on a weekend after her son fell a week earlier at the zoo.
A fever cropped up and mysterious red stripe materialised, too.
That stripe turned out to be a sign his wound had become infected, which prompted a visit to the doctor who diagnosed sepsis, a blood infection that can turn deadly if left untreated.
"It isn't something you can 'leave' until Monday when the doctors are back in the office," UK mum Alexandra Ruddy shared on Facebook.
Ruddy said she posted the story and the photo of the red steak as a warning to other parents--especially because his arm "wounds didn't look infected" and aside from the late developing fever, her son acted fine.
It has now been shared 40,000 times.
"I kind of knew, but doubted myself if that makes sense," Ruddy told USA TODAY about the sepsis diagnosis.
She almost didn't share
Ruddy said her husband "convinced" her to posting her son's story.
She felt "a bit silly" even taking him to the doctor on a Sunday though something in her told her not to wait. The doctor confirmed it and commended the mother for recognizing it and addressing it ASAP, she wrote on Facebook.
Her son was diagnosed with sepsis, with signs and symptoms that include fever, high heart rate, confusion, pain and shortness of breath. Red streaks are not directly related to sepsis but are a sign of spreading infection.
The doctor was caution not to alarm Ewan and told Ruddy to watch carefully over the next two days, his mum told USA TODAY.
"He did say if (the streak) gets longer in the next 48 hrs to go to hospital...bearing in mind Ewan was with me so I'm sure he wouldn't want to scare (him)!"
Ewan was sent home with a week-long regimen of antibiotics and made a full recovery after missing two days of school.
Same thing happened to a friend
Ruddy said it was friend who alerted her two years ago that red streaks could mean danger.
Her husband, the friend and her own gratitude that her son is healthy led her to share on Facebook not ignore red streaks or mum gut instincts.
"Thankfully the antibiotics are working and he is well in himself!" she wrote. "If you spot this red line running from a wound along the vein get yourself/your child seen straight away. Hopefully my post might help someone the way my friend's post from 2 years ago helped me."