Transmission-based precautions are implemented in addition to standard precautions when the route of transmission is not completely interrupted by standard precautions alone. Three categories exist based on the mode of pathogen transmission. Contact precautions prevent transmission of organisms spread by direct (skin-to-skin) or indirect (contaminated surfaces/equipment) contact. The epidemiologically important organisms requiring contact precautions include MRSA, VRE, C. difficile, scabies, and multidrug-resistant gram-negatives. Droplet precautions prevent transmission of pathogens carried in large respiratory droplets (>5 micrometers) generated by coughing, sneezing, or talking that travel up to 3-6 feet before settling due to gravity. Organisms include influenza, pertussis, meningococcal meningitis, and group A streptococcal pharyngitis. Airborne precautions address pathogens carried in airborne droplet nuclei (<5 micrometers) that remain suspended in air currents for extended periods and can travel long distances through ventilation systems. Only three diseases require airborne precautions: tuberculosis, measles (rubeola), and varicella (chickenpox)/disseminated herpes zoster. Airborne isolation requires negative-pressure rooms (airborne infection isolation rooms, AIIR) with 6-12 air changes per hour exhausted to the exterior, and N95 respirators or PAPRs for healthcare workers.