W32/Tefo is a detection for parasitically infected executable files made to load a malicious DLL detected as W32/Tefo.dldr. It does not modify the file size of existing executable file(s) as 150 bytes of malicious code is inserted into unused segments of the file(s).

Once executed it loads and executes the tefo.dll file from the same folder.

More information can be found at this McAfee page.