Programs designed to create new computer viruses, worms, and Trojans. Constructors are known to exist for Windows and macOS.
Such software can be used to generate source code and object modules for malware, as well as malicious files themselves.
Some constructors have an interface for configuring a ready-made worm or Trojan, with options to choose the type of malware, obfuscation/anti-debugging features, and other functions.