Lifeline sectors (energy, water, transportation, communications)

Of the 16 sectors, CISA singles out four as "lifeline" sectors: energy, water, transportation, and communications. These are the sectors whose functions the other 12 sectors most directly depend on to operate at all; a hospital (Healthcare and Public Health sector) can't function without electricity, water, and communications, for example. This is a big part of why energy specifically draws so much attention in critical infrastructure security: a failure there doesn't stay contained to the energy sector.