Why does catarrh smell?

Catarrh is produced to rid your body of pathogens, infections which enter the body through one of the 3 main interfaces. E.g the respiratory system. When the liver and kidneys cannot kill the pathogen, catarrh is produced to trap the disease, then it can be blown out through the nose, or swallowed and destroyed by stomach acid. When the bacteria pathogens become concentrated and trapped in cattarh, it may appear thick and coloured. It may also develop a smell due to the dead pathogens caught up in it. Hope this helps.