In this article we will discuss about:- 1. History of Infectious Diseases 2. Meaning of Infectious Diseases 3. Characteristics 4. Infectious Diseases Cycle 5. Control.
History of Infectious Diseases:
The classical study of infectious diseases was conducted by a British physician, John Snow, between 1849 and 1854. During this period a series of outbreaks of cholera occurred in London.
He found that cholera was spread by drinking water from a pump that was contaminated with raw sewage containing the pathogenic microorganisms. Thereafter, the pump was removed. Consequently the number of cholera cases remarkably declined. The second example is the outbreak of “Typhoid Marry” between 1896 and 1906 in New York City.
Meaning of Infectious Diseases:
An infectious disease is any change in a state of normal health in which part or the whole body of an individual does not function properly due to the presence of an infectious agent or its products. The phenomenon of growth, multiplication and establishment of an infectious agent in host tissues or within the cells in known as infection.
When a microorganism or agent lives on expense of the host i.e. derives nutrients for its growth, it is called parasite. However, if a parasite or its products cause(s) disease, the former is known as the pathogen. The ability of a pathogen to cause disease is termed as pathogenicity. The series of events involved for proper establishment of a pathogen in host cells/tissue (i.e. the process of disease development) is known as pathogenesis.
Epidemiology (epi means upon, demos means population, logy means study) is the science that deals with occurrence, determination, distribution and control of a disease. An individual who studies the epidemiology is called epidemiologist. When a disease occurs occasionally and at irregular intervals in a human population, it is known as sporadic disease e.g. typhoid.
A disease maintaining a steady, low level frequency at a regular interval is called endemic disease e.g. common cold. However, a sudden increase in occurrence of a disease beyond a limit is called epidemic (upon the people). If the occurrence of a disease increases within a large population over a wide region, it is called pandemic (pan means all).
Frequency of a Disease:
Frequency of a disease refers to its repeated occurrence as fractions in a given population. To measure the frequency, the epidemiologists use statistics and find out the rate of increase over the pre-existing cases! It is measured as an increase over per 100 or per 1000 individuals.
By measuring frequency one can speculate how severe a disease is? However, it is also related to morbidity or mortality. Morbidity is the number of individuals becoming ill by a specific disease within a susceptible population during a defined period.
It is measured as below:
Morbidity rate = Number of new cases of a disease during a specific period in a population/ Total number of individuals in the population.
Similarly, mortality rate refers to death of individuals due to a specific disease with respect to size of population of sufferers with the same disease.
It can be measured by using the following formula:
Mortality rate = Number of deaths due to a given disease/Size of total population of sufferers with same size.
Characteristics of Infectious Diseases:
The infectious diseases have characteristic signs and symptoms. Signs are objective changes in body, for example fever. On the basis of fever a disease can be recognised. Symptoms are the subjective changes for example pain, loss of appetite, etc. which are felt by the patients.
In a broad sense symptom is used for sign as well. In addition, a disease syndrome includes a set of signs and symptoms due to a particular disease; for example an AIDS patient experiences disease syndrome.
Moreover, the characteristic symptoms of a disease develop during certain phases. The knowledge of the phases helps in recognition of a disease. For example, incubation period which refers to time required after infection to the appearance of signs/symptoms. Incubation period varies organism to organism. Second is the prodromal stage i.e. the period during which there is onset of signs and symptoms of a disease.
It cannot be clearly found out. Third, the period of illness which is a phase during which the disease gets fully established and becomes most severe with characteristic signs and symptoms. The immune system is triggered. The last characteristic phase is the period of decline when signs and symptoms disappear and the disease is recovered gradually. This stage is known as convalescence.
Development of immunity in a large percentage of a population resisting infection and spread of pathogen is called herd immunity. A large proportion of susceptible population is immunized at a time by the Public Health Officials just to maintain high level of herd immunity.
The increase in susceptible individual is constantly monitored. Because new individuals are added in a given population due to birth and migration. The continuous monitoring helps in prevention of spread of infection agents and its survival in the patients.
Generally the pathogens do not change their nature because they are continuously transferred from one individual to the other and perpetuate in susceptible individuals. In addition, there are some pathogens that go on continuous changes and cause new epidemics, for example AIDS, influenza virus and Legionella bacteria. This feature of pathogens is called antigenic shift which is genetically determined major character of the pathogens.
Due to antigenic shift, they are not recognised by the immune system of the host. For example, antigenic shift in influenza virus occurs due to hybridization between two antigenic types, or two different influenza viruses (i.e. serovars) or animal virus and human virus. Sometimes smaller antigenic changes also occur in pathogens just to escape from immune system of the host.
These smaller changes occurring in pathogen time to time are called antigenic drift. When resistance in a given population is so high (herd immunity), the pathogen cannot infect humans. In such situation, these infect animals.
Moreover, due to antigenic shift or drift again the population of susceptible individuals increases. In this situation the Public Health Officials have to make sure that about 70% of individuals must be immunized so that the herd immunity could be maintained.
Infectious Diseases Cycle:
The infectious disease cycle is the chain of events that include epidemiological story of the infectious agents. Since infectious organisms perpetuate in hosts and are transmitted through vectors, knowledge of disease cycle helps in control of the disease.
The following aspects are linked with disease cycle:
(i) Sources of Disease:
A source is the site or location from where the pathogens spread to a new host either through environmental factors (e.g. soil, water, air, food) or indirectly through animate (animals or humans).
Reservoirs are the natural environment (soil water or air) or susceptible animal hosts where pathogens survive. A list of pathogens surviving in animals is given in Table 23.1.
Table 23.1 : Non-human reservoirs of infectious agents.
Carriers are the individuals already infected with pathogens. Humans are the most important carriers of certain pathogens. Four types of carriers have been recognised: active carriers (who have an overt clinical case of disease), convalescent carriers (who have recovered from the infectious disease but contain pathogens in sufficient number), healthy carriers (who harbour pathogens without being affected), and incubatory carriers (who incubate pathogen into a large number without falling ill).
Transmission of Pathogens:
For perpetuation of disease and survival of the pathogen, transmission from one host to other occurs by any of four main routes: air borne, contact, and vehicle and vector-borne.
(i) Air-Borne Transmission:
The pathogens remain suspended in air and are transmuted through droplet nuclei i.e. small particles (1-4 µm diameter) left from evaporation of large particles (10 µm diameter).
The droplet nuclei remain in air for hours or days and carried to individuals because the pathogens cannot grow in air. Examples of some air borne diseases are chickenpox, flu, measles, mumps, viral pneumonia, diphtheria, pneumonia, tuberculosis, meningitis, etc.
(ii) Contact Transmission:
Some of the pathogens spread when contact of host is done with the reservoir of pathogen. In other words contact refers to person-to-person contact through touching, kissing, or sexual contact. The diseases that spread through contact are herpes and boils (through contact of oral secretions or body lesions), infection of staphylococci (by nursing mothers), and AIDS and syphilis (through placenta or blood to blood contact).
(iii) Vehicle Transmission:
Vehicle refers to inanimate materials such as utensils, towels beddings, surgical materials, needles, food, water, etc. Bacteria spreading through food and causing food poisoning are Staphylococcus, Bacillus cereus, E. coli, Vibrio choleme. Salmonella typhi, Clostridium difficile, etc.
(iv) Vector-Borne Transmission:
A living organism that transmits a pathogen is known as vector such as vertebrates (e.g. dogs, cats, bats, goats, sheep, etc.) or arthropods (e.g. fleas, mites, insects, ticks, etc.). For example, flies carry Shigella on their feet from faeces to food materials.
Moreover, when pathogen does not undergo morphological and physiological changes within the vector it is called harborage transmission e.g. the plague pathogen. Yersinia pestis. When the pathogen undergoes morphological changes within the vector, it is called biological transmission e.g. Plasmodium vivax.
Control of Infectious Diseases:
Since the infectious diseases are spread by several agents and cause epidemics, it can be controlled by one or several measures. This involves the breaking of the links of disease cycle, eliminating the reservoirs of disease and making the individuals resistant (i.e. immunization).
(i) Breaking the Links of Disease Cycle:
The pathogens survive on animate or inanimate for sometime where from transmitted to suitable host. Therefore, if links between two stages of disease cycle are broken, further spread of the pathogen does not occur.
This includes general sanitation methods:
(a) Pasteurization of milk,
(b) Destruction of vectors by spraying insecticides (e.g. thiodon, malathion, etc.),
(c) Chlorination of water supply,
(d) Inspection of food and individuals handling it, etc.
(ii) Elimination of Source of Infection:
The source of infection can be eliminated by:
(a) Adopting quarantine (legal prohibition of entry of goods, animals, etc. from one country to other or one state to other within a country) and isolating the carriers,
(b) Destruction of animal reservoir (e.g. the cattle infected with foot and mouth disease virus are killed in other countries),
(c) Treatment of sewage (to check water borne transmission of pathogens), and
(d) Use of chemicals by individuals to eliminate the pathogens.
(iii) Immunization of Individuals:
For increasing the level of herd immunity, mass immunization programmes were launched. For example during 1960’s in India mass immunization programme of children against chicken pox was done. Secondly, Pulse Polio Immunization was launched in December 1995.
Thereafter, this programme is conducted twice a year in India. However, at International level, several programmes have been launched by the World Health Organisation (WHO), for example AIDS, etc.