On my son's last day of preschool he came home with a running nose that quickly turned into green snot. My infant daughter soon after came down with green snot as well. I figured it's likely just a cold because both kids played just fine and the only bother was really me wiping their noses so often. The big question was how long should I wait to find out if it is something worse?
After two weeks of this nonstop green snot my son finally came down with a fever and all the classic signs of an ear infection. He woke up from nap in a lot of pain so I acted quickly, I gave him some pain reliever to get his fever down and then made an appointment with his pediatrician for just a little over an hour later. What's great is that this time he was able to tell me that his ear hurt unlike the first time he had an ear infection.
I decided to make an appointment for my daughter as well because I was concerned hers might turn into an ear infection as well... point of clarification though, ear infections are not contagious. Thankfully, we were able to get back to back appointments and stay in the same room where both kids were diagnosed with what I expected: my son had 
an ear infection and my daughter had a sinus infection.
I don't prefer to medicate unnecessarily, neither child was showing signs of getting better so both kids were prescribed antibiotics. My son was given a generalized antibiotic while my daughter, who has never been on any medications, was given amoxicillin. They are 5 days into them and both are feeling much better but still have 5 more days on the medicine.
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