Foul smell coming from air/heat. smells moldy, makes people cough from the stench. not sure how to fix

When the heater or AC were turned on, a mildewy, moldish type smell came out of all of the vents. We had them cleaned out (by two different mechanics) and were advised to spray something through them. It helped for a very short while, but it came back. We kept trying and the same thing would happen. Eventually my dad ended up spraying household lemon-scented cleaner in it and it still didn't work.

when either the heat or air conditioning is activated, there is a foul, sickening, decaying, bacteriological smell that emanates from the vents that has caused sore throats, coughing, and sickness. i have flushed out the vents with detergent, anti-bacteriological agents, high- pressure car wash solution, but to no avail. the problem will not go away.