Alexandra Ruddy, 41, became so concerned about the blemish on little Ewan’s skin she dashed him to an out-of-hours doctors’ surgery.
And though she felt “a bit silly”, Alexandra’s decision was praised by the medics.
The youngster was diagnosed with sepsis, which kills around 15,000 people in England annually.
Now Ewan has made a full recovery after taking antibiotics.
He had initially hurt his hand one week before the diagnosis after a fall during a school trip to a farm.
Alexandra, who is a director of a law firm, has been praised for her quick thinking.
“Oh my goodness, well done to you for getting it checked and for highlighting it, hope he’s all ok,” one mum on Facebook wrote.
“Gosh, well done Alex for spotting it and getting medical attention. Glad Ewan is recovering well,” another added.
Speaking today from her home in Jersey, Alexandra said: “I wasn’t actually that shocked, but just didn’t think it was as serious as that.
“My friends told me to make people aware as the wounds weren’t very gunky. A couple of them wanted me to share it and it just snowballed from there.
“It’s an example of social media being used for good. Ewan was at school on Monday and then took part in a triathlon the following week.”
She has shared photos of Ewan’s mark on Facebook. They show a small red line going from the bottom of Ewan’s forearm and stretching to his wrist.
There is also a black mark on the red line which was made by the doctor and told Alexandra if the mark increased width, to bring him back.
Alexandra’s post has been shared more than 35,000 times and also racked up more than 14,000 likes.
The mum said: “A week or so ago the littlest fell over at the zoo. He took quite a bashing but once we got home I cleaned him up.
“The wounds didn’t look infected- they’d got bigger so I was concerned but they weren’t gunky etc. Yesterday on our way to the beach he showed me his hand.
“I wasn’t happy as I noticed red tracking down his vein. I then checked his elbow – the same. I took him down to the out of hours feeling a bit silly but when the doctor saw it he commended me on recognising it and getting down ASAP.
“This is blood poisoning/ sepsis. It isn’t something you can ‘leave’ until Monday when the doctors are back in the office.”