- Biologists don't define viruses as "alive" because they need the machinery of a host cell to replicate; they cannot do so on their own. Consequently, viruses reproduce by hijacking other cells and co-opting the cell's resources to make copies of themselves. Viral surface proteins bind to receptors on the surface of your cells, essentially tricking your cell into admitting the virus as if it were legitimate cargo. Once inside, the viral DNA or RNA directs synthesis of proteins that cause the cell to mass-produce new viruses, which may bud from its surface or be scattered when the cell bursts. This process eventually leads to the death of the host cell.
- Many viruses are specific for certain types of cells in their host; this specificity is called tissue tropism. The damage caused by the virus depends on its life cycle and the types of tissues it infects. The polio virus, for example, can infect and destroy cells of the nervous system; these nerve cells are difficult if not impossible to replace, and so the damage is irreparable. Common cold viruses infect the lining of nose and throat, which can be more readily replaced by your body, so the damage is not permanent. HIV infects cells of the immune system, specifically lymphocytes; as these cells are destroyed, the host's immune system ultimately breaks down, thereby leaving your defenses weakened against other invaders. In each case, a different type of damage is caused by infecting different tissue types.
- Many of the symptoms associates with sickness, such as coughing, sneezing and fever, are actually responses by your own body to the invaders. Your internal temperature, for example, is controlled by your hypothalamus, which ramps up your temperature in response to signaling molecules secreted by immune cells. Typically the immune system brings the infection back under control through various mechanisms. Killer T lymphocytes, for example, destroy infected cells, while B lymphocytes with receptors specific for the viral pathogen proliferate and produce antibodies that target the virus. In some situations, however, the virus is able to overwhelm the immune system and kill the host, as with ebola. In other cases, the immune system overreacts, and the overreaction is what actually causes death, not the virus. Many of those who die of the flu, for example, succumb to a so-called cytokine storm, where a vicious cycle of escalation by the immune system leads to potentially fatal inflammation and lung damage.
- Some viruses exhibit a more unusual life cycle; these pathogens infect the host cell but do not initially replicate out of control. Instead, they remain silent inside the host cell, replicating along with it when it divides. Later on, a trigger event may cause the virus to start replicating and destroy the host cell, but for much of its life cycle, the viral infection remains latent or hidden. These kinds of viral infections may not even cause any symptoms, but in some cases, they can dramatically increase the risk of certain kinds of cancers. One example is the human papilloma virus; latent infections by some strains of HPV can lead to cervical cancer.
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