EPO (Entry-Point Obscuring) virus
Nowadays Entry-point obscuring viruses are become more interesting and famous. Because it is a very difficult to detect, disinfect and remove. Even after the virus is fully understand it's a very difficult-to-detect and difficult-to-disinfect.
How simple viruses works:
When a virus infects a file, it must find some way to attain control and be executed. They simply change the entry-point of the infected application(PE files) and make it point to the virus body. So virus activity is very easy to detect, as it usually result in files whose entry-point resides outside the code section, and therefore marked as suspicious by a virus scanner.
EPO Virus Technique:
The EPO technique was developed to avoid the virus scanner detection. An entry-point obscuring virus is a virus that doesn't get control from the host program directly. Typically, the virus patches the host program with a jump/call routine, and receives control that way.
EPO is the most dangerous technique to develop the virus.