What Causes Coughing, Sneezing, Congestion With A Runny Nose?

I have a 3 1/2 month old boy who has been coughing, sneezing, and has a runny nose. This has been going on for 3-4 weeks. He was put on amox. for 10 days and didn t get better nor did he get worse. Now he s taking allegra and his congestion seems to be getting worse.