Infectious diseases

Definition-Although the term “infectious diseases” might be logically be applied to any illness which results from invasion of the body by a microorganism, its use is customarily restricted to those diseases which spread by direct contact from person to person.At this time we should not be concerned with diseases like pneumonia or bacterial endocarditis which though due to infection can safely be nursed in general wards without special barrier precautions.
The occurance of illness in human is the result of the complex interactions between the responsible agent(exposure) the host and the environment( the epidemiological triad).
The virulence of an organism depends on its infectivity(ability to colonise) and its pathogenicity(ability to damage).
Diseases which are associated with a long incubation period are more difficult to control. The first lines of host defence against microorganisms are the physical and chemical barriers: these include the skin, mucosa, normal secretions, internal sphinchters and the resident endogenous flora.
Initially, microorganisms elicit a non-specific inflammatory host response: the recruitment of polymorphonuclear cells and the elaboration of cytokines are important components of this response. The macrophages of the reticuloendothelial system(splenic and alveolar macrophages, hepatic Kupffer cells, renal masangial cells and CNS microglia) play an important role in this response.
An organism specific immune response is also mounted by the host: activation of T and B lymphocytes culminates in immunoglobulin production and complement activation. Cell mediated immunity plays a ritical role in cambating intracellular bacteria.
Bacteremia refers to the presence of viable bacteria in blood. Infection refers to the presence of a microorganism in the body with a local inflammatory response. Sepsis refers to the presence of microorganism in the body with a systemic inflammatory response. Severe sepsis refers to sepsis associated with organ dysfunction, perfusion abnormalities or hypotension. Hypertension which is the main problem at present.