Population-level and environmental measures that prevent or contain disease spread rather than treating or testing one patient at a time. This includes disease-control measures during an outbreak (contact tracing, cordon sanitaire, isolation in health care, quarantine, universal precautions), vector and parasite control at the population level (vector control broadly, mosquito control, deworming, mass drug administration, directly observed treatment short-course for tuberculosis), harm-reduction and community distribution programs (needle and syringe programmes, oral rehydration therapy as deployed in public health campaigns), environmental infection control (selective photonic disinfection), and the level-of-prevention framework, quaternary prevention, used to classify public health strategy itself. A measure delivered to and protecting one individual patient, such as a vaccine, a screening test or a prophylactic drug, is grouped elsewhere even when it is also used at population scale.