In 1971, Bob Thomas of BNN technologies wrote a program, 'Creeper System' - which was a self-replicating program to test Jon von Neumann's theory, which was published as an article in 1949. Since 1987- when a virus infected ARPANET, a vast network used by the Department of Defense and many universities, many antivirus programs were developed, which periodically checks the computer system in search of viruses.
Boot Sector Virus : Boot virus or boot sector virus alters the boot sector program stored in the hard disk or any other storage device such as floppy disks. It replaces the boot sector program with its own malicious version. It infects the computer only when it is used to boot up the computer. If it enters after the boot-up process, it will not infect the computer. For example, if someone forgets to remove the infected floppy disk when the pc is turned off and then turns on this pc, it runs the infected boot sector program during the booting process.
Macro Virus : Macro virus alters or infects the macros of a document or data file. It is embedded as a macro in a document and adds its codes to the macros of the document. The virus spreads when infected documents or data files are opened in other computers.
Resident Virus : The resident virus stays permanently in the primary memory (RAM) of the computer. When you start the computer, it becomes active and corrupts the files and programs running on the computer.
Non-resident Virus : Unlike the resident virus, the non-resident virus does not reside in the memory of a computer. So, it is not executed from the computer's memory. For example, executable viruses.
Worms : are self-replicating viruses that don't attach themselves to other files and lead to adverse effects on the host's computer.
Trojan Horse : A Trojan horse or “Trojan,” is a malicious bit of attacking code or software that tricks users into running it willingly, by hiding behind a legitimate program.
CMOS Virus : It infects the CMOS (Complementary Metal-Oxide Semiconductor) and is a memory chip that contains the system configuration. This virus can erase or reset the system configuration.
Encrypted Virus : Encrypted Virus its payload to make its detection more difficult. It comprises two parts: an encrypted virus body and a decryptor, which decrypts the virus when it is executed. After decryption, the virus can execute itself in order to replicate and become a resident. Furthermore, it is different from cryptolocker, which is a computer virus that encrypts the hard drive data and holds it for ransom.
Polymorphic Virus : Polymorphic viruses encrypt themselves differently every time these viruses attack the host computer. Since they use different encryption algorithms and change their format every time they affect the victim, they became untraceable and difficult by antivirus to detect them using signatures or string searches. Examples are Marburg, satan bug, etc.
Multipartite Virus: Multipartite Virus is spread and replicate in many different ways. Their actions depend on the OS installed and the presence of some particular type of files.
Here are many warning signs or symptoms which show that a computer is infected with a virus, Mainly slow performance of computer, Hard Drive issue, Impaired security solutions, Frequent pop-ups, Unnecessary advertisement, Network issues, Display problems, Affected Applications, etc,.