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Mucus is a normal content of the stool and should not cause you anxiety
if there is no blood in it and child has no complaints.
15 days old diarrhea
episode can not cause excess mucus output now.
The respiratory infection needs attention and decongestant medicines should be started orally along with the anti-allergic regimen till the kid is better.
I hope the advise would be informative and useful for you.