Unlike worms, viruses often require some sort of user action (e.g., opening an email attachment or visiting a malicious web page) to spread.

Next question that arises in our mind is that why should we bother about the viruses or in other words

What does a virus do?

A virus is simply a computer program -- it can do anything that any other program you run on your computer can do. Some viruses are designed to deliberately damage files, and others may just spread to other computers.

The virus that affects your computer in today's era are:

1. Trojan Horses

2. Worms

3. Bacteria

Another category in which we can subdivide the viruses are:

1. Virus that need Resident System or Host or some code to work upon

2. Virus that works and spreads on its own

Basic Definitions of various kind of viruses present today:

A worm is a type of virus that can spread without human interaction. Worms often spread from computer to computer and take up valuable memory and network bandwidth, which can cause a computer to stop responding. Worms can also allow attackers to gain access to your computer remotely.

A Trojan horse is a computer program that is hiding a virus or other potentially damaging program. A Trojan horse can be a program that purports to do one action, when in fact, it is performing a malicious action on your computer. Trojan horses can be included in software that you download for free or as attachments in email messages.

One important thing to note is that even many anti virus softwares fail to remove Trojan horses from your system. So, best solution to protect your computer from being affected is visit only the sites which are secure, if you receive email from an email-id unknown to you avoid opening that email of the attachments of which you are not sure. Another solution can be scan your system regularly and most important scan whatever external media you insert in your laptop before using it.