Most viral infections do not result to disease development with the exception of AIDS and some haemorrhagic viral infections such as Ebola and Lassa fever that leads to serious disease in the affected host and even death. The effectiveness of the immune system of the viral-infected host is one singular factor there is that can significantly affect the outcome of a particular viral infection in an individual. Even though the immune system may not eliminate totally the infecting virus, a strong immune system helps to contain the pathogenicity or virulence of a given pathogenic virus. The administration of potent vaccines and other antiviral agents to affected hosts and other susceptible host can help to treat the disease or infection and even prevent the contamination of a particular viral infection respectively.
Vaccine development (which is mainly based on the isolation or cultivation, attenuation or inactivation and the direct or indirect injection of a susceptible host with killed, live or purified subunits of causative pathogenic microorganism) has saved mankind from the onslaught of some life-threatening infectious diseases such as measles and smallpox through the control and the complete eradication of these diseases in some cases. The timely vaccination of a large population of susceptible host against a particular viral infection helped to minimize the spread of the pathogen in the immunized populace via herd immunity. And vaccination/immunization has helped to increase the life expectancy of mankind through the protection of the population from life-threatening diseases especially those caused by pathogenic viruses.
Viral infection unlike other microbial infections stimulate an immune response in the host; and this serves to protect the host from immediate attack and even from futuristic viral infection through specific immune responses. Pathogenic viruses as obligate intracellular microorganisms engage in uniquely intimate host-parasite relationships with the living organisms (plant, animal or man) that they infect, and this is due in part to the fact that viruses only exist or replicate in living cells. In the course of their pathogenicity or virulence in a particular host, pathogenic viruses express gene products that act to circumvent one or more of the several antiviral defense mechanisms (e.g. production of interferons) developed by the host organisms.
Nevertheless, the host resistance to viral infections involves both the humoral immunity and cell-mediated immunity. While some viral infections (e.g. influenza and common colds) can be contained by the host’s immune system; some others such as the causative agent of AIDS (i.e. HIV) overpowers the host’s immune system and makes it incapable to fight against the invading viral agent. Infections with some pathogenic viruses may lead to apoptosis; and this programmed cell death is a host defense mechanism that can be inhibited by some viruses. Antibodies produced by humoral immunity can neutralize pathogenic viruses by interfering with their attachment to host cells; and the production of antibodies also enhances the destruction of viral particles via phagocytosis. However, antibodies cannot completely eliminate the infecting virus once the virion has incorporated its genome into that of the host.
The cell-mediated immunity is one of the major important arms of the immune system that interfere with viral replication in vivo. Activated lymphocytes including cytotoxic T lymphocytes (CTL), helper T cells (CD+4) and cytotoxic T cells (CD+8) can recognize and destroy viral infected cells especially when these organisms bud off from their host cells. The production of interferons (which are protein substances produced by cells during viral infection) helps to reduce the spread of virus especially in some benign viral infections such as influenzae and cold. They stimulate the production of natural killer (NK) cells and T cells; and interferons also accelerate the immune response of a host to viral infection. And by acting on other effector cells of the immune system, interferons (which are antiviral cytokines) generally reduce the susceptibility of other uninfected cells of the host to the invading virus, and they do so by localizing the pathogenic virus so that they do not easily spread in the host’s body.
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