AC smells bad when air is blowing thru vent

1999 F150 4.6 ltr auto 87,431 ?Just a few weeks ago when I first noticed this real bad smell comming from the vents when using the ac. It smells like spoiled food. Is there a filter that maybe dirty from all the use it gets and I do smoke in the truck, but I do keep this truck clean and you can eat off the engine its so clean!If you can help me in this matter my nose will greatly appreciate it.

thank you
lloyd 59