The three most important characteristic of an infectious disease that determine whether or not the infectious disease will be transmitted are infectivity, pathogenesis and virulence.

My justification for picking the above three characteristics are as follows
Infectivity This is the most important factor that determines if an individual will have an infection. In every individual, there is a particular dose of the pathogen that is required to be transmitted to the host before the host can be said to have an infection.  If the infecting dose is less than is required for infection, there will be no disease except for immune compromised or immune deficient individuals. The pathogen has to successfully break through the different defence mechanism of the body and multiply in the hosts system before significant infection can be present. The infectivity of a pathogen is therefore important in considering the outcome of an infection from an exposure.

Pathogenicity This is the ability of the agent to cause disease in an infected host. A lot of non-pathogenic organisms that are part of the normal flora are not capable of causing disease provided they remain at their normal site. They therefore have insignificant pathogenicity. Significant pathogenicity therefore depends on the ability of the particular pathogen to cause infection due to its spread from the original of exposure. For example, multiplication of the bacteria that are part of the normal bacteria flora of the gastrointestinal tract, skin, etc., is generally not considered an infection on the other hand, multiplication of pathogenic bacteria (e.g., Salmonella species)  even if the person is asymptomatic  is deemed an infection (Brooks, Carroll, Butel  Morse, 2007).

Virulence This is an indicator of the severity of the infection. It is otherwise a measure of the degree of pathogenicity of a pathogen. A highly virulent organism can cause infection at a relatively low dose compared to a pathogen that is less virulent. Some factors confer high virulence on some pathogens and these include the presence of capsules that inhibit phagocytosis, production of toxins that are capable of causing tissue damage etc. Capsules enable bacteria to be more virulent because the macrophages and neutrophils are unable to phagocytize the encapsulated buggers (Gladwin  Trattler, 2006). Therefore, the degree of virulence of the pathogen that an individual is exposed to is important in determining if the immune system will effectively mount an effective response against the pathogen.

The three other characteristics that I consider vital but less important to the above three are toxigenicity, resistance and antigenicity.

Toxigenicity It is not all pathogens that have to produce toxins before they can cause an infection. Organisms can act directly andor through their toxins (Boon, Colledge, Walker  Hunter, 2008). The production of toxins is mainly peculiar to bacteria (especially gram negative bacteria). Bacteria may produce two types of toxins called exotoxins and endotoxins (Todar, 2009).  Toxigenicity is therefore less important in determining infection since a lot of pathogens mediate their actions without producing toxins.

Resistance The resistance of an organism which is simply a measure of its ability to survive under adverse environmental condition is mainly important when the appropriate treatment has to be administered. The pathogen has therefore already colonised the host and caused infection. The resistance determines the kind of treatment that should be given. For example, many strains of pathogens such as bacteria, due to frequent use of a particular antibiotic develop resistance to the antibiotic. An antibiotic sensitivity test is therefore required to determine the particular antibiotics that the organism will be sensitive to.

Antigenicity Not all pathogens induce significant   production of antibodies by the humoral part of the immune system. Antigenicity is used to determine the possibility of reinfection if the host is exposed to the pathogen again. In this case, it is only if the individual is exposed to the pathogen the first time that there is a high risk of infection since the body promptly responds through the production of antibodies.


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