Infection and immunity
Infection and immunity involve interaction between the host (man) and the infecting microorganism. Based on their relationship to their hosts, microorganism can be classified as saprophytes and parasites.
Saprophytes are free-living microbes the subsist on dead or decaying organic matter. They are found in soil in water and play an important role in the degradation of organic materials in nature.
Parasite : Parasite may be defined as a microorganism, which lives on a living host and derives nutrition from the host, without any benefit to the host.
Commensal : Commensals (con, with; mensa, table, i.e. living together) live in complete harmony with the host body without causing any harm to it. The commensals constitute the normal bacterial flora of the body such as staphylococcus epidermidis of skin and Escherichia coli of gastrointestinal tract. The commensals subsist on secretions, food residues or waste products of the body.
Opportunistic pathogens : Some commensals or saprophytes can produce disease when the body resistance is lowered; such organisms are termed as opportunistic pathogens.
Pathogens : A microorganism capable of producing disease is called pathogen. Majority of the pathogenic bacteria are heterotrophs.
Pathogenicity : Pathogenicity refers to the ability of a class of bacteria to produce disease.
Virulence : Virulence is a measure of the degree of pathogenicity and depends on invasiveness and toxigenecity of the organism.
The term infection and disease are often used synonymously when discussing host-parasite relationship. The difference between these terms is helpful to understand the development of the normal micro-flora, which represents a successful host-microorganism relationship.
Colonizer : A microorganism that makes contract with the host but does not cause the host to produce am immune response that would destroy it or cause an allergic response called a colonizer.
Infection : Microorganisms that make contact with the host and elicit an immune response (antibody production) are said to cause infection.
Disease : If the microorganisms brings about an abnormal condition in the host, in addition to elicting antibodies, it is said to cause a disease.
Infection : Infection may be defined as lodgement and multiplication of an infections do not invariably results in disease. Some infections may remain asymptomatic and other may lead to development of signs and symptoms after breakdown of host-parasite relationship in favour of the parasite. It is necessary to distinguish between the term infection and infection disease. Infection does not always results in disease. In fact disease is but a rare consequence of infection, which is a common natural event.
Infection may be classified in various ways such as :
Primary infection : Initial infection with a parasite in a host is termed as primary infection.
Reinfection : Subsequent infections by the same parasite in the host are termed as reinfection.
Secondary infection : When a new parasite sets up an infection in a host whose resistance in lowered by a pre existing infections disease, is termed as secondary infection.
Focal infection (focal sepsis) : A localized site of infection from which bacteria and their products are spread to other parts of the body. OR
Indicates a condition where, due to infection or sepsis at localized sites such as appendix or tonsils, generalized effect are produced.
Cross infection : When in a patient already suffering from a disease a new infection is set up from other host or another external source, it is termed as cross infection.
Nosocomial infection : Cross infections occuring in hospitals are called nosocomial infections.
Iatrogenic infection : It refers to physician included infection resulting from investigative, therapeutic or other procedures.
Depending on whether the source of infection is from the host's own body or form external sources, infections are classified as endogenous or exogeneous respectively.
Based on the clinical effects of infections, they may be classified into different varieties.
Inapparent infection : Is one where clinical effects are not apparent.
Sub-clinical infection : It is a synonym of inapparent infection.
Atypical infection : In one in which the typical or characteriastic clinical manifestations of the particular infections disease are not present.
Latest infections : Some parasites, following infections, may remain in the tissues in a latent or hidden from proliferating and producing clinical disease when the host resistance is lowered. This is termed latest infections.
Sources of infection : Infection may be acquired endogenously or exogenously.
Endogenously sources : This occurs with microorganisms of the host's normal flora which behave as pathogens outside their habitat.
Exogenous sources :
Man :
The commonest source of infection for human beings themselves. The parasite may originate from a patient or a carrier. A carrier is a person who harbors the pathogenic microorganism without suffering form any ill effect because of it. Several types of carriers have been identified. A healthy carrier is one who harbors the pathogen but has never suffered form the disease caused by the pathogen. while a convalescent carrier is one who has recovered form the disease and continues to harbor the pathogen in his body. Depending on the duration carrier state lasts less than six months, while chronic carriage may last for several years and some times even for the rest of ones life. The term contact carrier is applied to a person who acquires the pathogen from a patient, while the term paradoxical carrier refers to a carrier who acquires the pathogen form another carrier.
Animals :
Many pathogens can infects both man and animals. Animals may, therefore act as sources of infection. In some instances the infection in animals may be asymptomatic, such animals serve to maintain the parasite in nature and act as reservoir of human infections. They are, therefore, called reservoir hosts. Infectious diseases transmitted from animals to human beings are called zoonoses. Zoonoses are infections between vertebrate animals and man. Many of those infections affect agricultural workers and veterinary surgeons. General public acquires infection through contaminated meat and milk. An epidemic occurring in animal is called epizootic (plague in rats), where as the term enzootic refers to endemic occurring in animals (plague in rats and brucellosis in cattle). Some of the examples of zoonotic diseases are : bacterial (plague from rats), Viral (Rabies from dogs), Protozoal (Toxoplasmosis from cats), Fungal (Zoophilic dermatophytes from cats and dogs).
Insects :
Blood sucking insects may transmit pathogens to human beings. The disease so caused are called arthropod borne disease. Insects such as mosquitoes, ticks, mites, flies and lice that transmit infections are called vectors. Transmission may be mechanical (transmission of dysentery if thphoid bacilli by the domestic fly). Such vectors are called mechanical vectors. In other instances, the pathogen multiplies in the body of the vector often undergoing part of its developmental cucle in it.
Such vectors are termed as Biological vectors (Aedes aegypti mosquito in yellow fever, Anopheles mosquito in malaria). Biological vectors transmit infection only after the pathogen has undergone a developmental cycle . The interval between the time of entry of the pathogen into the vector and the vector becoming infective is called the extrinsic incubation period.
Beside acting as a vector, some insects may also act as reservoir hosts. Infection such insects by transovarial or transstadial passage.
Soil and water :
Some pathogens can survive in the soil for very long periods. Spores of tetanus bacilli may remain viable in the soil for several decades and serve as the source of infection. Fungi are also parasites such as roundworm and hookworm survive in the soil and cause human infections.
Water may act as the source of infection either due to contamination with pathogenic microorganisms (cholera vibrio, infective hepatitis virus) or due to the presence of aquatic vectors (cyclops in guinea worm infections).
Food : Contaminated food may act as a source of infection. The presence of pathogens in food may be due to external contamination (food poisoning by staphylococcus) or due to pre-existent infection in meat or other animal products (salmonellosis).
Methods of transmission of infection :
The routes of transmission may be by way of contact, vehicle, vector, air and transplacental. The infective agent has to find out a portal of entry for successful parasitism.
Contact :
Infection may be acquired by contact which may be direct or indirect. Direct contact refers to transmission of microorganisms from person to person by close personal association. Handshaking, kissing, sneezing, coughing and sexual contact represent the most usual ways that microorganisms are transferred by direct means. Sexually transmitted disease such as syphilis and gonorrhea illustrate spread by direct contract . The term contagious disease had been used for diseases transmitted by direct contact. Where as infectious diseases are those signifying all other modes of transmission. The distinction is now not generally empolyed.
Sneezing and coughing may also be considered a method of direct spread , provided the individuals are within a few feet of each other. The microorganisms are expelled in droplets that are carried only a few feet and then drop to horizontal surface. Direct spread of this kind is characteristic of measles, a viral disease.
Indirect contact may be through the agency of fomites, which are inanimate object such as clothing, pencils or toys that may be contaminated be a pathogen from one person and act as a vehicle for its transmission to another. Pencils shared by school children may act as fomites in the transmission of diphtheria and face towels in trachoma.
Inhalation :
Respiratory infections such as influenza and tuberculosis are transmitted by inhalation of pathogen. Such microbes are shed by the patients into the environment, in secretions from the nose or throat during sneezing, specking or coughing. Large drops of such secretions fall to the ground and dry there. Pathogens resistant to drying may remain viable in the dust and act as sources of infection. Small droplets, under 0.1 mm in diameter, evaporate immediately to become minute particles or droplet nuclei (Usually 1-10 m in diameter) which remain suspended in the air for long periods, acting as sources of infection.