Tall Man Plague

The Tall Man Plague is a 100%-fatality virus introduced to humanity by the Tall Man in order to weaken their resolve in fighting him. It starts with ordinary flu symptoms, but ultimately the human's head expands several times and explodes, infecting everything within the blast radius.

Phantasm IV: Oblivion
When Mike briefly visits the deserted future Los Angeles, he is escorted away by Jody, who warns him about a risk of infection if he stays.

Phantasm V: Ravager
When Reggie reunites with Mike within the post-apocalypse reality, the latter reveals to him that the Plague's extreme effectiveness and the scientists' inability to cure it led to total governmental collapse, allowing the Tall Man to rule the world.

Appearances

 * Phantasm IV: Oblivion (1998) - Mentioned only
 * Phantasm V: Ravager (2016)