4. Convalescent period (recovery, death, or carriage). Convalescent carrier is one who has recovered from the disease and continues to harbor the pathogen in his body. Depending on the duration of carriage, carriers are classified as temporary and chronic. (Temporary carrier state lasts less than 6 months; chronic carriage may last for several years and sometimes even for the rest of one’s life).
Some of infectious diseases can spread within the population. Communicable(contagious) disease is that may be transmitted from an infected host to uninfected individuals readily and rapidly.
All the infectious diseases cause the immune response in the host and some of such diseases can cause the development of the acquired immunity (see LESSON 15).
¨ Types of infectious diseases
Infectious diseases may be classified according to the different features. Types of infections are as follows.
Depending on whether the source of infection is from the host’s own body or from external sources, infections are classified as endogenous or exogenous respectively.
Endogenous disease is caused by opportunistic, normally harmless microbes inhabiting the body. It usually occurs in the immunocompromized host as a result of autoinoculation (transfer of microbes indigenous to one area of the body to another area where they may cause disease).
Exogenous infections may have different sources of infection (a place or places where the pathogen normally lives and multiply):
v Anthroponoses – are infections, for which the human is the source (a patient or a carrier).
v Zoonoses – are infections for which the animals are the source of infection.
v Sapronoses – occur where pathogens are able to survive, multiply and grow in the inanimate environment (for example, in soil and water).
NOTE: Anthropozoonoses – both human and animals may be the source of infection. Saproanthroponoses – both human and environment may be the sources of infection, etc.
Depending on how the infectious diseases spread in the community, they may be classified into different types:
v Endemic – are those which are constantly present in a particular area or in the particular population. These diseases occur with constant frequency in population. (For example, typhoid fever is endemic in most regions of India).
v Epidemic (epidemos = among the people)– sudden outbreaks of infectious disease within the community, it spreads rapidly involving many persons in an area at the same time. The number of cases of a disease is increased over that is considered to be the norm or standard.
v Pandemic – is a worldwide epidemic that spreads through many areas of the world involving very large numbers of persons within the short period.
v Sporadic are the diseases that occur occasionally without a predictable pattern.
Infections can be also classified as:
v Primary infection – is an initial infection with a parasite in a host
v Reinfection – is a subsequent infection by the same parasite in the host.
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v Secondary infection – is an infection, when in a host whose resistance is lowered by a preexisting infectious disease, a new parasite sets up an infection from another host or internal source. Secondary infection occurring in hospitals are called nosocomial (nosocomion = hospital) infection. Nosocomial infections are usually iatrogenic, that is infection which refers to physician-induced, resulting from investigative, therapeutic or other procedures.
v Coinfections (mixedinfections) develop following exposure to several different pathogens that have usually entered the body by traumatic injury. Gas gangrene, an extreme example of mixed infection, can be caused by as many as 10 coinfecting microbial species introduced during surgical or accidental trauma. Each type of microbe could cause disease alone or in combination with the other coinfectants.
NOTE: A chemotherapeutic agent must be effective against all participating pathogens, since a single species that escapes treatment may continue to cause disease.
Infectious diseases may be localized (superficial or deep-seated), or generalized.
Generalized infection involves the spread of the infecting agent from the site of entry by contiguity, through tissue spaces or channels, along nerves, lymphatics, or through the bloodstream.
v Septicaemia is the condition where bacteria circulate and multiply in the blood, form toxic products, and cause high swinging type of fever.
v Pyaemia is the condition where pyogenic bacteria produce septicemia with multiple abscesses in internal organs (such as spleen, liver, kidneys)
v Toxaemia is the condition where bacterial exotoxin is in the blood and can cause different severe clinical symptoms. During this process toxigenic bacteria may stay in the site of entry without spreading within the body.
NOTE: Circulation of bacteria in the blood is known as bacteraemia. Transient bacteraemia is a frequent event and may occur even in healthy individuals. During some infectious diseases, bacteraemia of greater severity and longer duration is seen, but microbes do not multiply within the bloodstream.
NOTE: The terms viraemia, fungaemia, parasitaemia characterize the presence of viruses, fungi, or protozoa in blood (correspondingly).
Based on the clinical effects of infections, they may be classified into the following varieties:
v Inapparent infection – is one where clinical effects are not apparent (synonim is subclinical)
v Atypical infection – is one in which the typical or characteristic clinical manifestations of the particular infectious disease are not present.
v Latent infection – if some parasites, following infection, may remain in the tissues in a latent or hidden form, proliferating and producing clinical disease when the host resistance is lowered.
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v Acute disease – is a disease with a rapid onset and short duration.
v Chronic disease – is a slowly progressing disease of long and often indeterminable duration.
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Elements essential for the occurrence of infectious process within the population are as follows:
1. Susceptible macroorganism (host)
2. Potentially harmful infectious agent (microbe)
3. Environmental conditions.