There are several types of malicious items that can infect a computer not only viruses but other forms of malicious software, some of which I have listed here.
1. Boot Sector- This type of infection infects the mbr or master boot record and this will allow the infection to run immediately when the computer is booted. or it could disable your operating system altogether.
2. Multi-Partie viruses have different effects based upon the files, software etc already on the infected computer and the types of symptoms can vary.
3. Browser Hijacker- this type of infection could be classified as adware it will redirect your browser pages especially on searches and your homepage to place advertising on your computer for the purpose of gaining revenue for the infection creators. This tyoe of infection could be from a voluntary download or attached to a free program that you downloaded.
4. Website scripting- this type of infection can come from a malicious website or from a legitimate website that has been hacked and had code injected into it's already complex code even without the website owners knowledge
5. File Infector- The most common type of virus is a file infector virus it attaches it's self to an exectable file .exe etc... or overwrites an exectuable file and when the file or program is run the virus code executes also.
6. Ransomware- Malicious software that wants a you to pay money or a ransom to remove it such as the FBI virus or a fake antivirus that shows you several fake infections on your computer and wants you to purchase their program.