The ability of a pathogenic virus to cause disease in a host (i.e. viral pathogenesis or virulence) is usually affected by several factors which are both of viral origin and host origin. For a given pathogenic virus to cause infection or disease in a host, the virus must invade the host cell, establish itself and initiate the processes that leads to the development of the disease it is known to cause; and the disease development is usually deciphered as mild or severe clinical symptoms in the host – depending on the virulent nature of the infecting pathogenic virus. The pathogenesis and/or virulence of a given pathogenic virus are dependent on the individual host infected, the dosage of viral administration as well as the particular viral species and strain involved in the invasion. The interactions of these factors mentioned above and as shall be discussed in this section will determine the nature of the infection or disease to be produced. They will also determine whether or not there will be clinical symptoms and signs of the disease in the affected host. Some of these important factors that generally determine or affect the degree of viral infection in a host are highlighted in this section.


Immunity is the ability of a host’s body to resist the development of a disease or infection cause by a pathogenic microorganism. It can be innate immunity (non-specific immune response) or acquired immunity (specific immune response). Humoral immunity (immune response that involves antibody production) and cellular immunity (immune response that involves the production of T cells inclusive of cytotoxic T cells and helper T cells) are the two major types of immunity in the body of a host; and these forms of protection are involved in the control and prevention of disease establishment by a given pathogenic virus. Thus, a strong and intact immune system is critical for the protection of a host against viral infection. Macrophages, phagocytes, and cytokines such as interferons (IFN), interleukins (IL) and tumor necrosis factor (TNF) are components of a host immune system which cooperatively attack or ward-off the untoward effects of invading pathogenic viruses in the body of a host. Nevertheless, some pathogenic viruses have developed innovative ways of surmounting the immune system of their host cells or organisms; and the mutation (a change in the genetic makeup of an organism) of some viral strains – which have allowed mutant forms of some pathogenic viruses have given impetus to this phenomenon.

Pathogenic viruses (such as the influenza viruses and coronaviruses that cause viral flu and severe acute respiratory syndrome, SARS respectively) undergo mutation to change their antigenic structure or proteins, and this development allows them to remain masked within their host – so that they will remain undetected by the host immune system while causing or establishing disease in the individual. In addition, some pathogenic viruses such as the human immunodeficiency virus (HIV) which causes AIDS (acquired immunodeficiency syndrome) have the ability to weaken the host immune system to an extent that it becomes less functional in attacking other pathogens that invades the host. HIV infects the cells of the immune system (particularly the T helper cells) and thus deprives the host immune system to carry out its normal function of protecting the body from the attacks of pathogens that invades it to cause disease.


The route of viral spread is another factor that affects the pathogenicity of a given pathogenic virus. Pathogenic viruses that infect humans are mainly spread via blood inclusive of the serum and plasma. Semen, saliva, lymph fluids and the cerebrospinal fluids are other host’s materials through which a pathogenic virus could be transmitted from one individual to another. Because blood attains a general circulation in the body, it is often the most common means via which pathogenic viruses can be spread within a defined human host.


A pathogenic virus must be highly infectious before it can establish an infection or disease in its host. Infectivity is the ability of a pathogenic virus to enter a host cell and thus initiate the process of diseases development. Pathogenic viruses with higher infectivity produces more sever clinical symptoms of the disease than viruses with lesser form of infectivity. A pathological state can only be established with a virus and its host in vivo when the invading pathogenic virus is infectious in nature.


The dosage of pathogenic viruses that invaded a given host is another factor that affects the development of disease as well as the pathogenicity or virulence of the invading pathogen. The higher the infectious dose of the pathogenic virus in the host organism, the more intense the production of viral progeny. New virions are critical for the maintenance or sustenance of the viral infection in the host; and when new viral progeny are not turned out via active viral replication, the viral disease process will be halted and possibly broken. The production of low viral progeny may also be associated with the low infectious dose of the invading pathogenic virus.

When the pathogenic virus that invaded a particular host cell fails to produce new virions or viral progeny within then host cell, the infection is said to be abortive. But when new viral progeny are produced after invasion within the infected host cell, the infection is said to be a productive viral infection. However, some pathogenic viruses may assume a latency phase – during which they do not produce new viral progeny after invasion of the host cell. But for such latent viral infections, the production of new virions is activated at a later stage in the host; and this is because such pathogenic viruses still posses the ability to initiate productive viral infection at a later time during the disease or infection process.


The location of susceptible host cells for viral invasion within the host is another important factor that affects the pathogenicity and/or virulence of pathogenic viruses. The pathogenicity of a given pathogenic virus may not be fully established if the target organ, tissue or cell of the virus within the host is susceptible to the invading virus. After invasion or entry, pathogenic viruses must be able to locate and reach their specific sites within the host and thus bind specifically to the receptors on the host cells. This process of invasion and attachment to receptors on the host cell is a critical step in viral replication mechanism. It is noteworthy that the closer the proximity of the invading pathogenic virus to its target site or receptor within the host, the lesser the time it will take the pathogen to reach its target organ and thus establish the disease process.


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Alan J. Cann (2005). Principles of Molecular Virology. 4th edition. Elsevier Academic Press,   Burlington, MA, USA.

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Barrett   J.T (1998).  Microbiology and Immunology Concepts.  Philadelphia,   PA: Lippincott-Raven Publishers. USA.





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