The disorder that often causes a foul smelling nasal discharge is sinusitis.
Sinusitis is the inflammation of your sinuses and is often caused by viruses, bacteria or even fungi.
The bacteria, fungi or viruses that can cause sinusitis can cause thick, discolored and foul smelling mucus.
Other conditions that causes a foul smelling nasal discharge are nasal polyps, dental infections, Vasomotor rhinitis, Foreign objects, Rhinolith and even certain medications can all be causes of foul smelling nasal discharge.
Certain medications like nasal sprays and decongestants can irritate your nasal passages and cause a foul smelling discharge to occur.
Rhinolith is a hard stone like formation in your nasal passage which can trap in bacteria and cause a bad odor and foul smelling nasal discharge.
Vasomotor rhinitis is a condition that causes excessive nasal mucus production that can also become foul smelling and lead to foul smelling nasal discharge if not properly cleaned.
Objects that get trapped in your nose such as small toys, cotton balls etc can cause a foul smell as they decompose and lead to foul smelling nasal discharge.
Dental infections such as infections in the gums and teeth can produce gases which can travel up your nasal passages and cause you to experience foul smelling nasal discharge.
Even infections in the throat could lead to foul smelling nasal discharge.
Most cases of foul smelling nasal discharge are nothing to worry about and clear up on their own within a few days to a few weeks.
However if you continue to experience foul smelling nasal discharge you should see a doctor.
In the mean time you could use some saline nasal spray to clean out your nose regularly to keep bacteria out of the nose and prevent bacteria from building up in the nose.