An infectious disease is a clinically evident disease of humans or animals that damages or injures the host so as to impair host function, and results from the presence and activity of one or more pathogenic microbial agents, including viruses, bacteria, fungi, protozoa, multicellular parasites, and aberrant proteins known as prions. Transmission of an infectious disease may occur through several pathways; including through contact with infected individuals, by water, food, airborne inhalation, or through vector-borne spread.
A contagious disease (also called a communicable disease) is an infectious disease that is capable of being transmitted from one person or species to another. Contagious diseases are often spread through direct contact with an individual, contact with the bodily fluids of infected individuals, or with objects that the infected individual has contaminated.
The term infectivity describes the ability of an organism to enter, survive and multiply in the host, while the infectiousness of a disease indicates the comparative ease with which the disease is transmitted to other hosts. an infection however, is not synonymous with an infectious disease; as an infection may not cause clinical symptoms or impair host function.
Overview
Among the almost infinite varieties of microorganisms, relatively few cause disease in otherwise healthy individuals.[4] Infectious disease results from the interplay between those few pathogens and the defenses of the hosts they infect. The appearance and severity of disease resulting from any pathogen depends upon the ability of that pathogen to damage the host as well as the ability of the host to resist the pathogen. Infectious microorganisms, or microbes, are therefore classified as either primary pathogens or as opportunistic pathogens according to the status of host defenses.
Primary pathogens cause disease as a result of their presence or activity within the normal, healthy host, and their intrinsic virulence (the severity of the disease they cause) is, in part, a necessary consequence of their need to reproduce and spread. Many of the most common primary pathogens of humans only infect humans, however many serious diseases are caused by organisms acquired from the environment or which infect non-human hosts.
Organisms which cause an infectious disease in a host with depressed resistance are classified as opportunistic pathogens. Opportunistic disease may be caused by microbes that are ordinarily in contact with the host, such as bacteria or fungi in the gastrointestinal or the upper respiratory tract, and they may also result from (otherwise innocuous) microbes acquired from other hosts (as in Clostridium difficile enterocolitis) or from the environment as a result of traumatic introduction (as in surgical wound infections or compound fractures). An opportunistic disease requires impairment of host defenses, which may occur as a result of genetic defects (such as Chronic granulomatous disease), exposure to antimicrobial drugs or immunosuppressive chemicals (as might occur following poisoning or cancer chemotherapy), exposure to ionizing radiation, or as a result of an infectious disease with immunosuppressive activity (such as with measles, malaria or HIV disease). Primary pathogens may also cause more severe disease in a host with depressed resistance than would normally occur in an immunosufficient host.
Agents and vectors
Infectious disease requires an agent and a mode of transmission (or vector). A good example is malaria, which is caused by Plasmodial parasites, chiefly Plasmodium falciparum but does not affect humans unless the vector, the Anopheles mosquito, is around to introduce the parasite into the human bloodstream.
The vector does not have to be biological. Many infectious diseases are transmitted by droplets which enter the airway (e.g. common cold and tuberculosis).
Clearance and immunity
Infection with most pathogens does not result in death of the host and the offending organism is ultimately cleared after the symptoms of the disease have waned.[4] This process requires immune mechanisms to kill or inactivate the inoculum of the pathogen. Specific acquired immunity against infectious diseases may be mediated by antibodies and/or T lymphocytes. Immunity mediated by these two factors may be manifested by:
* a direct effect upon a pathogen, such as antibody-initiated complement-dependent bacteriolysis, opsonoization, phagocytosis and killing, as occurs for some bacteria,
* neutralization of viruses so that these organisms cannot enter cells,
* or by T lymphocytes which will kill a cell parasitized by a microorganism.
The immune response to a microorganism often causes symptoms such as a high fever and inflammation, and has the potential to be more devastating than direct damage caused by a microbe.
Resistance to infection (immunity) may be acquired following a disease, by asymptomatic carriage of the pathogen, by harboring an organism with a similar structure (crossreacting), or by vaccination. Knowledge of the protective antigens and specific acquired host immune factors is more complete for primary pathogens than for opportunistic pathogens.
Immune resistance to an infectious disease requires a critical level of either antigen-specific antibodies and/or T cells when the host encounters the pathogen. Some individuals develop natural serum antibodies to the surface polysaccharides of some agents although they have had little or no contact with the agent, these natural antibodies confer specific protection to adults and are passively transmitted to newborns.
Diagnosis and therapy
The field of infectious diseases also occupies itself with the diagnosis and therapy of infection.
Diagnosis
Diagnosis is initially by medical history and physical examination, and imaging (such as X-rays), but the principal tool in infectious disease is the microbiological culture. In a culture, a growth medium is provided for a particular agent. After inoculation of a specimen of diseased fluid or tissue onto the medium, it is determined whether bacterial growth occurs. This works for a number of bacteria, for example Staphylococcus or Streptococcus.
Certain agents cannot be cultured, for example the above-mentioned Treponema pallidum and most viruses. The first serological markers were developed to diagnose syphilis (the Wassermann test, later replaced by the VDRL and TPHA tests). Serology involves detecting the antibodies against an infectious agent in the patient’s blood. In immunocompromised patients (e.g. AIDS), serology can be troublesome, because the antibody reaction is blunted.
A more recent development is direct detection of viral proteins and/or DNA in blood or secretions. This can be done by PCR (polymerase chain reaction), involving the amplification of viral DNA and its subsequent detection with anti-DNA probes.
The classification of infectious disease
One way of proving that a given disease is “infectious”, is to satisfy Koch’s postulates (Robert Koch), which demand that the infectious agent is identified in patients and not in controls, and that patients who contract the agent also develop the disease. These postulates were tried and tested in the discovery of Mycobacteria as the cause for tuberculosis. Often, it is not possible to meet some of the criteria, even in diseases that are quite clearly infectious. For example, Treponema pallidum, the causative spirochete of syphilis, cannot be cultured in vitro – however the organism can be cultured in rabbit testes].
Epidemiology is another important tool used to study disease in a population. For infectious diseases it helps to determine if a disease outbreak is sporadic (occasional occurrence), endemic (regular cases often occurring in a region), epidemic (an unusually high number of cases in a region), or pandemic (a global epidemic).
Therapy
When a culture has proven to be positive, the sensitivity (or, conversely, the antibiotic resistance) of an agent can be determined by exposing it to test doses of antibiotic. This way, the microbiologist determines how sensitive the target bacterium is to a certain antibiotic. This is usually reported as being: Sensitive, Intermediate or Resistant. The antibiogram can then be used to determine optimal therapy for the patient. This can reduce the use of broad-spectrum antibiotics and lead to a decrease in antibiotic resistance.
The work of an infectiologist
Doctors who specialise in the medical treatment of infectious disease are called infectiologists or infectious disease specialists. Generally, infections are initially diagnosed by primary care physicians or internal medicine specialists. For example, an “uncomplicated” pneumonia will generally be treated by the internist or the pulmonologist (lung physician).
The services of the infectious disease team are called for when:
* The disease has not been definitively diagnosed after an initial workup
* The patient is immunocompromised (for example, in AIDS or after chemotherapy);
* The infectious agent is of an uncommon nature (e.g. tropical diseases);
* The disease has not responded to first line antibiotics;
* The disease might be dangerous to other patients, and the patient might have to be isolated.
The work of the infectiologist therefore entails working with patients and doctors on one hand and laboratory scientists and immunologists on the other hand.