Coughing up a small amount of blood when someone has an upper respiratory infection and a cough is not uncommon. The infection can cause irritation of the upper airways, the area at the back of the nose and sinuses. The blood could be from the irritation of the upper airways. This irritation will go away when the infection is gone. It could even be from a series of strong coughs. If this subsides and goes away, it
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Don't panic! There are various things that it could be, the first one which comes to my mind in an otherwise healthy child with a cold is possibly blood from his irritated nasal passages had gotten down the back of his throat and come up with his mucous. This frequently happens when my children have colds, as their allergies always worsen and nosebleeds are more frequent as well. I would suggest you call your son's pediatrician and mention it and follow the doctor's instructions. Best of luck & I hope he's better soon!
Starion
Since it was a very small amount of blood mixed in with mucous, it should be ok. As the last poster said, it could be from a nasal issue.
If you spot bright red blood, then its fresh blood from a nearby source. Dark blood is older blood and typically less worrysome. If it was a one time thing, there is little to worry about.
If you are concerned, you may want to take your son to your general practioner for tests. This might help put you and your son's mind at ease.
Best,
Anai Rhoads