How often do people become ill from unknowingly consuming tainted marijuana? what could side effects be?

Hard to know: Marijuana can be adulterated with all kind of things -- from pesticides to other hallucinogens. Symptoms depend on the particular "extra" substance, on top of marijuana's ability to induce psychosis all by itself in some. How to measure the frequency of this adulteration? Marijuana is usually sold far from the place it was grown & packaged, so answers are elusive. Another reason for user beware.