The ability to check a program

A virus must also be able to check a file that will ditulari, for example, 
he served infect program extention: . doc, he has to check whether 
document file has been infected or not, because if it is, akan 
useless invected again. This is very useful to improve the ability of a 
virus in the case of speed invected a file / program. The public be 
by viruses is to have / give alert in the file / program that has been infected so easy to recognize by the virus. Sample designation is for example, provides a unique bytes in each file that was infected.