Primords were apelike proto-human beasts created as a result of an ordinary person touching the hot green ooze brought up from the Earth's interior by the Inferno Project's drill. A stricken individual would become disoriented and develop green skin and hairy hands. They would become bestial, violent and inhumanly strong, and their body temperature rose to such a degree that anything they touched burned. As time went on, the afflicted person would become hairier and more monstrous, growing green beards and wild green hair.
The length of time needed for the transformation varied from person to person. In normal temperatures, a Primord would only gradually transform. But when in extreme heat, the transformation took only a few seconds. Individual Inferno staff members who became Primords included Professor Eric Stahlman, Harry Slocum, John Bromley and Private Wyatt of UNIT.
Primords preferred heat and cold could either subdue or kill them. Although they could die by ordinary means, they were very hard to kill. They would attack normal humans and their heated touch against bare skin could cause another person to start becoming a Primord as well. All of the Primords that came about as a result of Inferno were either subdued or killed; Slocum was shot and Wyatt fell off of a catwalk to his death, whilst Stahlman and Bromley were taken down using fire extinguishers.
Primords existed in the alternate universe as well, due to the Republic of Great Britain undertaking its own version of the Inferno Project. All of the individuals who had been turned into Primords or were turning into Primords in the Doctor's universe were already Primords when he arrived, even those who'd died in the other universe. Slocum was already dead, and Wyatt was shot by his fellow Republican Security Force troops. Bromley was captured, but killed a guard and escaped before being sprayed with a fire extinguisher by the Doctor. Stahlman remained at large and turned many other Inferno scientists into Primords under his command, as well as Platoon Under Leader Benton.
Stahlman himself was shot multiple times by Brigade Leader Lethbridge-Stewart, but survived before being ultimately subdued with a fire extinguisher by Greg Sutton, and all of the Primords he created died when the alternate version of Earth was destroyed in a fiery apocalypse.