There are basically three completely different 'types' of viruses. They are animal, plant and bacterial viruses. Each type of viruses is independent of the others. Meaning that a plant virus can not be transmitted to a bacterium and such. So far, there has never been any type of life found on Earth, which is not susceptible to viruses. Some species have more than one hundred different viruses plaguing their kind.
Within each classification, viruses are specific to a certain kind of cell. Viruses tend to be very picky about their hosts. They also have preferred ways of entrance into their hosts. A virus' method of entry is very specialized, and it is one of the main ways a virus is able to locate it's victims. Take, for example, a virus that targets host cells located in the stomach. If a person inhaled such virus particles they would not be harmed. On the other hand, if any virus molecules were ingested into the stomach, the hosts would immediately be infected. Some viruses even require cells to be in certain stages of life. These viruses may prefer actively dividing cells or cells that are younger.
Viruses use a simple marking process to identify the cells they attack. All viruses have special molecules on the outer covering that can search out and identify particles on cell surfaces. Every cell has a unique set of markers that identifies it to other cells. These surface molecules dictate the cells the each virus can recognize and infect. The interaction between virus surface and cell surface determines whether infection of the host will be successful or not. Host cells have to be of a very specific type or viruses will not be able to replicate and survive. All cells have surface receptors, which viruses use to identify the cell by their markers and if they match, attach themselves to it. Both sides have to be in good contact, and conditions have to be just right. When a virus securely attaches itself to a host cell in good condition, the infection begins.
Viruses do not possess any life sustaining characteristics, and do not require any nutrients. In fact, without a proper host viruses lie dormant indefinitely. Infection takes place when a virus comes in contact with its intended host. As soon as a virus encounters its victim, it attaches itself to the organism. Furthermore, most viruses prefer a certain type of host cell and a specific mode of entrance. Naked viruses, those without a structured casing, directly enter the cells while other types of viruses fuse themselves to the outsides of their victims and inject their genetic material inside the cell. Once the genetic material of a virus is transferred to the host cell, the virus can 'take over' by incorporating its DNA into the hosts DNA much like they used to do in the prehistoric days. The infected cell is essentially a factory in charge of virus manufacturing. In a process called budding, mature viruses leave the cell a few at a time. Lysis is the much more devastating cousin of budding. In lysis, the cell membrane of the host is completely destroyed, killing the cell. The new viruses are unleashed instantaneously. Almost all viral infections result in the death of the host, but in rare cases viruses leave their host cells alive. When this happens the cells are normally damaged beyond repair. With each successive transmission between hosts a virus is able to replicate itself thousands of times, and ensure the continuance of its reign of terror.