To do anything, a virus needs your help. First, it needs to get into your body, and it does that in much the same way as bacteria does—by being inhaled into your lungs, swallowed into your digestive tract, or absorbed through a cut in your skin.
Once inside your body, the virus drifts aimlessly until it comes into contact with the right cell—one that has a coat of proteins that complements those of the virus. When the virus bumps up against this cell, its proteins lock on. (Keep in mind this isn’t a conscious decision for the virus—it’s simply a reaction caused by the fact that it fits the target cell like a fuzzy sweater and a strip of Velcro.)
What happens next is more unsettling. The virus launches the multiple steps of its attack procedure. First, the virus needs to get inside the cell. In some cases, the cell may engulf the virus in the same way that it swallows tiny nutrients, pulling it in. Or the virus may inject its genetic material through the cell wall. Either way, the damage is done. The foreign genetic material finds its way deep into the cell’s working parts, where it quickly takes over.
In the classic case, the virus inserts a short snippet of DNA into the target cell (as shown in step 1 of the figure on page 221). Like all strands of DNA , this DNA contains instructions for building specialized proteins. The target cell cheerily follows these instructions, unaware that it’s helping the enemy.
Once built, these proteins begin to carry out their pre-programmed functions manufacturing thousands of new viruses. (This is what happens in step 2 and step 3.) In this way, the virus hijacks the inner workings of the cell, like a pirate commandeering an ocean liner. But all the while, the virus hasn’t actively done a single thing. It’s just a set of malicious instructions that your body executes, simply because the virus was in the right place at the right time.
Once they’ve taken over a cell, most viruses replicate like Viagra-fuelled rabbits. Eventually, they leak out of the cell through tiny pores or blow it apart like an overfilled water balloon (as you can see in step 4).
There’s a virus for virtually every type of cell. Viruses infect animals, plants, and even bacteria. (In fact, it’s likely that the bacteria that causes cholera would be completely harmless were it not for the presence of a toxinproducing virus embedded inside.) However, the viruses that affect one species are often unable to affect another, or they may have dramatically different effects. HIV is a well-known example—not only is it unable to infect other animals, but the related SIV strains that affect monkeys and chimpanzees rarely cause the compromised immune system and debilitating symptoms of AIDS.
Viruses don’t necessarily correspond to illnesses. In fact, many viruses have no symptoms. They don’t destroy their host cells, reproduce very quickly, or create poisonous compounds. Virtually all people have at least a few harmless viral passengers hiding in their bodies.
Source of Information : Oreilly - Your Body Missing Manual