Can anyone help me with a movie title

Hi I am new to the site and was wondering if any keen Sci Fi fans could help me. I am trying to find the title of a Sci Fi horror film I was shown as  a 11 year in the late 1980s by an uncle who recently passed away.

The film begins with two alien space craft in a dogfight and one is shot down, as it crashes it hits something and the camera pulls back to reveal its a human thumb and the aliens are microscopic. The aliens infect the human and make them immortal a little bit like nano bots except for their one weakness fire, that makes those infected explode. 

I can't remember much else except a couple being on the run and hunted I think by the government, overall the film has a body horror fell to it like  David Cronenberg movie.

PS it could have been Italian.