There are various types of infects:

Worms: these are programs on computers that get copied from one system to another

E-mail viruses: these are available in e-mail messages

Trojan horses: these claims to perform a certain function and does something different

Viruses: these are a small portion of the software that piggybacks on the real programs.

On the other hand, there are different working phases like:

Infection phase where the file is infected

Attack phase where the file that is infected is ruined and the actions are performed by the virus on the system.

Usually a virus is very dangerous for the system and they infect several things like:

Batch file viruses

Source code viruses

Companion virus: by simply installing a similar named COM file

Cluster viruses: it changes the directory of the DOS informations so that the entries of the directory point out the code of the virus instead of the real program.

Macro viruses

System sector viruses

Visual basic worms that are based on visual basic scripts 

Internet plays only a small role in spreading the viruses and only if any person send a document that is infected or a virus proned website is open then the virus gets into the system. It makes anonymous postings and also plays a big role in the future.

The best part is that there are good services and most of you may be unaware about it. Good virus has several advantages like:

The file compressor virus that compresses the files that it has infected

The anti virus virus that is located somewhere else other than the computer virus and remove them. The disk Encryptor virus that makes sure that the users data is secured

The maintenance virus that maintains the tasks.

These kind of good viruses are not usually available and most of the time our systems are attacked by normal and harmful viruses.

Download CSE PPT Topic on Computer Viruses And Antiviruses An Insight.