Viruses online and offline
and dealing with them when they infect
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Classes
 

There are several different classes of Virus. I have listed them below with examples in some cases.
 

    File Viruses

    infect ordinary *.EXE or *.COM files. Usually they just append the virus code to the file; but recent versions have
    gotten trickier, and hide their additions. Friday the 13th loads into memory on execution of the infected file, and if
    the date matches Friday the 13th, deletes *.exe files - often itself included!

    Multipartite Viruses

    infect both systemic areas such as boot blocks and executable files. These are opportunistic infectors, finding the
    available files at random.

    Systemic Viruses

    focus on the system files necessary for DOS. These are files which control the allocation of system resources, such
    as directories, and files. In some cases a much more basic level of attack against CMOS structures is attempted.

    Polymorphic Viruses

    combine a range of strategies to attack the integrity of the operating system.

    Stealth viruses

    try to conceal there presence. This may be as simple as modifying the file structure to conceal the additional code
    added to a file. It may go so far as making sure that when added to machine code in the *.COM file that the CRC is
    not changed (a technically very tricky bit of work).

    Meta Viruses

    Viruses that execute their nastywork in the very helpful meta languages embedded in powerful modern programs
    like MS Word.

    Trojan Horses

    are crude, front door attacks. They rely on simple naiveté (which I have fallen for many times). The level ot the
    threat can be very potent, however, because they do not require any backdoor - you gave them the key!

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