Final answer:
A used needle is disposed of in sharps containers because it is a biohazard that could harbor disease-causing organisms.
Step-by-step explanation:
A used needle is disposed of in sharps containers because it is a biohazard. A biohazard is any biological material that could potentially make someone sick, including disease-causing organisms. A used needle could potentially harbor blood contaminated with disease-causing organisms, making it potentially dangerous if not properly disposed of.