Want to know what happens after you've been headcrabbed? Take a look. If you or a companion are ever unavoidably headcrabbed, you would be well advised to put a bullet through the affected brain as soon as possible, for only massive and irreversible damage to cerebral tissue has any apparent effect on the headcrab's ability to "drive" its victim.
Its leap - if not dodged - is not precisely fatal to its prey, for once the headcrab has attached itself to the skull of a human host, there commences a swift and horrible process of "zombification" which gives the host a continued existence of the most objectionable sort.
They're the most hated, most annoying, and least understood of all xenofauna. The sharp limbs of the headcrab can easily tear through your hazard suit, if you're not careful. These guys love to dwell in dark corners and other out-of-sight areas, trying to surprise victims.
The lowly headcrab is the first enemy you'll encounter, and it's by far the most common. Know what you're dealing with and don't go into battle unprepared.