One of the instruments was found to be more contaminated with bacteria than the palm of a doctor's hand after being used to examine 71 patients.
Among the microbes spreading from patients was the potentially deadly superbug MRSA (methicillin-resistant Staphylococcus aureus).
Dr Didier Pittet, from the World Health Organisation Collaborating Centre on Patient Safety at University of Geneva Hospitals in Switzerland, said: "By considering that stethoscopes are used repeatedly over the course of a day and may harbour several thousands of bacteria collected during a previous physical examination, we consider them as potentially significant vectors of transmission.
"From infection control perspectives, the stethoscope should be regarded as an extension of the physician's hands and be disinfected after every patient contact."
Dr Pittet's team conducted a study in which 71 patients were examined by one of three doctors using sterile gloves and a stethoscope.
After each examination, the tube and diaphragm of the stethoscope and the physician's hands were checked for bacteria.
The stethoscope's diaphragm was more heavily contaminated than all regions of the hand except the fingertips.
In addition the stethoscope tube was covered in more bugs than the back of the doctor's hand.