Journal of Clinical Infectious Diseases & Practice
Open Access

An infectious disease is disorder caused by four different organisms such as bacteria, viruses, fungi or parasites which are also called as pathogens. Pathogens may enter into human body or host through broken tissue, eyes, nose and mouth.

Based on their relationship to their hosts, microorganisms can be divided as saprophytes (which depends on dead and decaying matter) and parasites (disease causing organisms).

Infectious disease may be classified as localized or generalized. A localized infection which is also called as systematic infection occurred in one body part or organ and can be categorized as superficial or deep seated.

Generalized infections are defined as the spread of the infecting organism from the site of entry by contiguity, through tissue spaces along the lymphatic system or through the bloodstream. Circulation of the bacteria in the blood is known as bacteremia, a condition in which the blood is infected. Transient bacteremia is harmless in healthy individuals and may occur during chewing, brushing of teeth or straining at stools. The actions of bacteria in the blood are immediately overtaken by phagocytic cells and are unable to initiate infection. Bacteremia of greater severity and longer duration is seen during generalized infections as in typhoid fever.

The bacteria in the blood which is called as Septicemia is defined as the condition in which bacteria circulate and multiply in the blood, form toxic products and cause high, fluctuating type of fever. Staphylococcus aureus, Streptococcus pneumonia, E. coli are the examples which cause Septicemia.

Depending on their spread in the community, infectious disease may classified into various types which include endemic, epidemic, pandemic and sporadic. Endemic disease is constantly present in a particular area. For example, typhoid fever is endemic in most parts of India. Epidemic disease spreads rapidly, involving many persons in that particular area at the same time period. Example include influenza which causes annual winter epidemics in cold countries. A pandemic is an epidemic that spreads through multiple areas of the world involving very huge numbers of people within a short period. Influenza, cholera, plague and enteroviral conjunctivitis are pandemic diseases. Epidemics vary in the rapidity of the spread. Water borne diseases such as cholera and hepatitis may cause explosive outbreaks while diseases which spread by person-to-person contact evolve more slowly. Such creeping and smouldering epidemics as that of cerebrospinal fever are termed as prosodemic diseases.

The term contagious disease had been used for diseases transmitted by direct contact, distinct from infectious disease, signifying all other modes of transmission. Respiratory infections such as influenza and tuberculosis are transmitted by inhalation of the pathogen. Ingestion infections are generally acquired by the ingestion of food or drink contaminated by pathogens. Infections transmitted by ingestion may be waterborne, foodborne (food poisoning) or handborne (dysentery). Pathogens, in some instances may be inoculated directly into the tissues of the host. Insects may act as mechanical or biological vectors of infectious disease. Infections may sometimes be transmitted during administration of injections, lumbar puncture and catheterization if meticulous care in asepsis is lacking. Some pathogens are able to cross the placenta barrier and infect the fetus in utero. This is known as vertical transmission. The outcome of an infection will depend on the interaction between microbial factors which predispose to pathogenicity and host factors which contribute to resistance.