Infectious Diseases

Infection refers to the successful transmission of a microorganism (bacterium, virus, fungus, or parasite) to the host, with subsequent multiplication of the microorganism (infectious agent). The host response to infections is highly variable and depends on the relationship between host and infectious agent. An infection may be subclinical (unapparent) or may result in disease. The disease is the clinical expression of the infection and indicates that the microorganisms not only are present and replicating, but that they are also disrupting the host to the extent that signs and symptoms are being produced. A sign (e.g., fever, rash, or vomiting) is the objective evidence of damage to the host, whereas a symptom (e.g., headache) is subjective evidence of damage to the host. A subclinical infection is marked by absence of visible symptoms, but there is a measurable host immune response, either through the appearance of specific antibodies or through cell-mediated reactions, such as positive tuberculin test results.

An individual who harbors a microorganism without evidence of disease and, in some cases, without evidence of a host immune response is called a carrier. The carrier status may take place during the latent phase of the incubation period as a part of asymptomatic disease, or may be chronic and persist following recovery from illness. Carriers may shed organisms into the environment intermittently or continuously, and this shedding may lead to transmission of the infectious agent. Shedding and potential transmission may be increased by other factors affecting the host, including infection by another agent.

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An infection is the product of three interrelated factors: (1) the agent, (2) the host, and (3) the environment.


The   infectious   agent   is   the   first   link   in the  chain  of  infection.  Factors  that  are  important in the characterization of the agent are as follows: (a) Infectiousness is the easiness with which a pathogen can spread in a population. (b) Pathogenicity is the ability of an agent to cause disease. An example of an organism with high pathogenicity is the smallpox virus. There is no known human carrier state for this agent; once infected with the virus, the host will develop disease. (c) Virulence is the pathogen’s power to cause severe disease and therefore is a measure of the severity of the disease. The virulence of organisms ranges from slightly to highly virulent. (d) Invasiveness is the ability of a microorganism to invade tissues. For example, Vibrio cholerae is not invasive, remains localized to the intestinal mucosa, and produces a toxin that is responsible for signs and symptoms. In contrast, Shigella is highly invasive and, by invading the submucosal tissue, becomes established and causes disease. (e) Infective dose is the quantity of the agent necessary to cause infection; it varies from microorganism to microorganism and from host to host and is influenced by the mode of transmission. In a study of typhoid fever, it has been demonstrated that with an inoculum of 103 Salmonella typhosa, no clinical disease develops in normal volunteers. When the dose of the inoculum is increased to 107 or to 109 bacteria, there is a 50% and 90% attack rate, respectively. The attack rate is the ratio of the number of new infections divided by the number of exposed, susceptible individuals in a given period. (f) Specificity refers to how microorganisms may be specific with respect to their range of hosts. Some salmonella species, such as S. typhimurium, are common to both animals and humans, but others have a narrow range of specificity; for example,  S.  dublin  primarily  infects  bovines,  while S. typhosa is known to infect only humans. (g) Antigenicity refers to the ability of the agent to induce immune responses in the host. Agents may or may not induce long-term immunity against infection. For example, reinfection with the measles virus is thought to be rare, whereas repeated reinfection with gonococci bacteria is common.


In order to cause disease, pathogens must be able to enter the body, adhere to specific host cells, invade and colonize host tissues, and inflict damage on those tissues. Entrance into the host typically occurs through mucosal membranes or through wounds. Although some pathogens can grow at the initial entry site, most invade areas of the body where they are not typically found. The human body is capable of immune  responses  that  reduce  the  likelihood  that an agent will penetrate and cause disease. The host immune responses may be divided into specific and nonspecific. Specific immune mechanisms include humoral (antibodies) and cell-mediated immunity. Nonspecific host defenses require the presence of intact skin and mucous membranes, which provide barrier protection. The high pH of our gastric juices is lethal to many agents that manage to enter the body via ingestion. Tears and saliva can be thought as means to wash away would-be infectious agents. Several other factors influence the susceptibility and host response to an infectious agent. As we age, the ability of our nonspecific defense mechanisms to fend off agents may decrease. Immunocompromised individuals are more at risk to develop infectious disease than normal individuals. Genetic traits may result in reduced or enhanced susceptibility to infection and subsequent disease. For example, individuals who lack both Duffy blood group antigens are resistant to malaria caused by Plasmodium  vivax,  as  these  antigens  are  required for attachment of the agent. Individuals who have a homozygous 32 base pair deletion in the CCR5 gene have been found to resist HIV-1 infection, as CCR5 is a co-receptor used by HIV-1 for attachment and entry to the cell. The nutritional status and dietary habit of the host are also important. Malnutrition contributes significantly to infectious disease-related morbidity in developing countries.


The environment refers to the domain in which the disease-causing agent may originate, survive, or exit. This milieu has been categorized into three areas: physical, describing the geography and the climate; biologic, made up of plants, animals, and other life forms; and socioeconomic, which is the totality of the behavioral and attitudinal characteristics of a group of individuals. All these environmental categories have an impact on both microorganisms and potential hosts, as the environment may either enhance or diminish the survival of infectious agents and plays a major role in bringing microorganism and host into contact with each other. The environment may act as a reservoir or niche that fosters the survival of the infectious agent.


Is the method by which an infectious agent passes from a source to the host. There are two general modes of disease transmission: direct and indirect. Direct transmission occurs through direct physical contact or direct person-to-person contact, such as touching with contaminated hands, kissing, or sexual intercourse. Vertical transmission—that is, the passage of the microorganism from mother to child—is considered a form of direct transmission.

Indirect transmission occurs when agents are transferred or carried by an intermediate source: vehicles, fomites, or vectors. Indirect transmission may be airborne (via droplets or dust particles), waterborne, vehicleborne, foodborne, or vectorborne. Airborne transmission occurs when a person sneezes, coughs, or talks, spraying microscopic pathogen-carrying droplets into the air that can be breathed in by nearby susceptible hosts (as occurs for the transmission of the influenza  virus). Airborne  transmission  also occurs when droplets are carried through a building by heating or air-conditioning ducts, or are spread by fans throughout a building or complex of buildings (as occurred for Legionnaire’s disease). Waterborne transmission occurs when a pathogen is present in drinking water, swimming pools, streams, or lakes used for swimming (as in the examples of cholera and shigellosis). Vehicleborne transmission relates to fomites, such as eating utensils, clothing, washing items, combs, shared drinking bottles, and so on. Foodborne illness is transmitted by ingestion of contaminated food or drink. This type of transmission may be active if the organisms replicate while in the vehicle (such as salmonellae in food), or passive if the organisms are passively carried by the vehicle, as in the example of the hepatitis A virus in food. Some vectorborne disease transmission processes are simple mechanical processes, as when the pathogen uses a host (e.g., fly, flea, or rat) as a mechanism for a ride, for nourishment, or as a physical transfer process in order to spread. This is called mechanical transmission. When the pathogen undergoes changes as part of its life cycle within the host/vector and before being transmitted to the new host, it is called biologic transmission. An example of biologic transmission is that of the Plasmodium protozoan that completes its sexual development cycle in the female Anopheles mosquito.

Prevention And Control

Infectious diseases can be prevented or controlled at a variety of points, depending on the infectious cycle for the particular disease. Direct person-toperson transmission may be inhibited by proper hygiene and sanitary conditions as well as education. Vectorborne diseases may be prevented by control measures that either kill the vector or prevent its contact with humans. Infection by a pathogen or development of a pathogen within a host may be prevented by vaccination. Finally, drugs may be used to prevent infection or suppress the disease process.


Centers  for  Disease  Control  and  Prevention,