Shadows of the Past by Richard Schiver

When Washington D.C Homicide Detective Sam Hardin is called to investigate a gruesome killing he stumbles upon the theft of an ancient alien dagger that had been discovered in the barren wastes of Antarctica. But this is no ordinary dagger. It has an odd effect on anyone it comes into contact with, for it contains the essence of an alien race that once ruled a young Earth.

Partially disabled Vietnam vet Jack Griffith has been counting down his final days as the cancer, compliments of his service in South East Asia, slowly consumes his body. While clearing a clogged storm drain he is accidentally stabbed by the dagger that had been stolen to repay a gambling debt. Surrounded by rats as death approaches he is given a second chance at life when that alien presence invades his body.