HLLO (High-Level Language Overwriting) is a category of overwriting viruses written in high-level programming language, usually Pascal, C, C++ or Basic.
Different variants in the HLLO family may have different behaviors and payloads.
The viruses' targets are usually the executables, they overwrite a file or a number of files during the runtime of the virus (non TSR). Programs infected by these viruses will no longer function properly, and the timestamp of the infected file are usually be changed to the time when the virus infects it.
But not every variant contains a payload.
There are 243 known variants under this category.
See separate article: List of HLLO viruses