Infectious diseases may be localized or generalized.
Localized Infections may be superficial or deep seated. Generalized infection involves the spread of the infecting agent from the site of entry by contiguity, through tissue spaces or channels, along the lymphatics or through the bloodstream.
Transient bacteremia is a frequent event even in healthy individuals and may occur during chewing, brushing of teeth or straining at stools. The bacteria are immediately mopped up by phagocytosis cells and are unable to initiate infection.
Bacteremia of greater severity and longer duration is seen during generalized infections as in typhoid fever.
Pyemia is a condition where progenitor bacteria produce septicemia with multiple abscesses in the internal organs such as the spleen, liver and kidney.
Depending on their spread in the community, infectious diseases may be classified into different types.
These are those diseases which are constantly present in a particular area. Typhoid fever is endemic in most parts of India.
An epidemic disease is one that spreads rapidly, involving many persons in an area at the same time. Influenza causes annual winter epidemics in the cold countries.
A pandemic is an epidemic that spread that spreads through many areas of the world involving very large number of people within a short period. Influenza, cholera, plague and enteroviral conjunctivitis are pandemic diseases.
Waterborne diseases such as cholera and hepatitis may cause explosive outbreaks, while diseases which spread by person-to-person contact evolve more slowly. Such creeping or smoldering epidemics, as that of cerebrospinal fever, are termed prosodemic diseases.
Text Book Of Microbiology