Computer viruses are transmitted through various means and spread very fast. The following are the most common ways how computer viruses are transmitted.
1. Running a virus-infected program, image, video or any other file.
2. Booting a computer from a virus-infected medium, such as USB drive, etc.
3. Opening of a virus-infected email attachment.
4. Running pirated software. Pirated software files are known for often having virus attached to them, but you may find a virus-free pirated software as well if you're lucky.
5. Mobile devices. If you have virus in your smart phone and you connect it to your computer to copy files. You will end up copying virus-infected files from your phone to computer.
6. Bad Intranet. If you're connected to a computer network (LAN), you may access a file from a computer in the network, which is virus-infected. This way, the virus will run on your computer.
7. Hackers. Some hackers can target computer directly and even can bypass your computer Firewall to send a virus in your computer and run it. Though this way of virus transmission is not as common as others, it does happen quite often everywhere in the world
8. Opening a virus-infected USB drive. Many a times a USB drive is infected by virus in such a way that the moment you open it to see its contents, the virus runs - without even needing you to execute any file that is in it.
These are the many ways how computer viruses are transmitted. By reading this, now you may have understood the importance of using antivirus software. Some people think that antivirus software is not needed. To these people, I explained earlier whether their computers are really safe without antivirus or not. I also explained in another article the most common effects of viruses on your computer. I recommend you also read this.