Here is the original de-compiled source code and executable for the Ambulance virus, a historical MS-DOS virus released into the wild in June 1990.

When working with all malware (old or new), exercise caution before running any potentially malicious code. This repository is not intended for any malicious use, but simply to act as an educational reference for malware researchers, educators, InfoSec students, and anyone interested in historical code.

Excerpt from Wikipedia:

Ambulance or Ambulance Car is a computer virus that infected computers running a DOS operating system in June 1990. It was discovered in Germany.