Due to the Headcrab's slow adaptation process possessing a human, the host tends to stumble towards their pray slowly, like a typical zombie. Headcrab Zombies possess the body of the average human, male or female, with a Headcrab latched to the host's head. Gordon Freeman first encountered these creatures a few minutes after leaving Test Lab C/33a. The conversion process is debatable, but most likely between five to thirty minutes. It should be noted that the human would still be alive at this point, and in excruciating pain. They would then leap onto the human's head and crack open the skull, and gain access to the human's brain, eventually taking over their entire nervous system. Upon entering Earth, Headcrabs roamed around Black Mesa, that is, until they encounter a human. Headcrabs were among the many races that invaded Earth. Multiple races, both militant and wild, poured only into Black Mesa, slaughtering and eating scientists, and civilians alike. On the month of May, the 16th, staff at the Black Mesa Research Facility had unintentionally opened a rift between Earth and Xen, causing a Resonance Cascade. The Nihilanth had noted scientists from the Planet Earth had constantly arrived at the point, and after they took in the crystal sample 'GG-3883', the Nihilanth readied it's forces for a full on invasion of Earth, as the sample was noted as the 'purest, but most unstable'. On an interdimension drop off point known as Xen, sometime after the Nihilanth had enslaved and took command of the Vortigaunts, the Nihilanth had created the Gonarch, a giant creature, in order to to produce miniature 'headcrabs'. They also appeared in the fan remake of the original game using Half-Life 2's source engine, Black Mesa. Headcrab Zombies are unconventional zombies first appearing in the 1998 first person shooter, Half-Life and its spi n-o ffs as well as se qu el s.

Half life poison headcrab