An infectious disease is a clinically evident disease resulting from the presence of pathogenic microbial agents, including pathogenic viruses, pathogenic bacteria, fungi, protozoa, multicellular parasites, and aberrant proteins known as prions. These pathogens are able to cause disease in animals and/or plants. Infectious pathologies are usually qualified as contagious diseases (also called communicable diseases) due to their potentiality of transmission from one person or species to another. Transmission of an infectious disease may occur through one or more of diverse pathways including physical contact with infected individuals. These infecting agents may also be transmitted through liquids, food, body fluids, contaminated objects, airborne inhalation, or through vector-borne spread.
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 important clinical symptoms or impair host function.
Among the almost infinite varieties of microorganisms, relatively few cause disease in otherwise healthy individuals. 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 pathogenic 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 colitis) 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.
One way of proving that a given disease is “infectious”, is to satisfy Koch’s postulates (first proposed by Robert Koch), which demands that the infectious agent be identified only in patients and not in healthy controls, and that patients who contract the agent also develop the disease. These postulates were first used in the discovery that Mycobacteria species cause tuberculosis. Koch’s postulates cannot be met ethically for many human diseases because they require experimental infection of a healthy individual with a pathogen produced as a pure culture. Often, even diseases that are quite clearly infectious do not meet the infectious criteria. For example, Treponema pallidum, the causative spirochete of syphilis, cannot be cultured in vitro – however the organism can be cultured in rabbit testes. It is less clear that a pure culture comes from an animal source serving as host than it is when derived from microbes derived from plate culture. 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).
National Integrated Food Safety Initiative
The purpose of the National Integrated Food Safety Initiative is to support competitive projects that address priority issues in food safety that are best solved using an integrated approach.
Integrated food safety programs in CSREES support multistate, multi-institutional, multidisciplinary, and multifunctional research, extension, and education activities. Special emphasis is given to research describing multifunctional activities (for example, research that contains research, education, and extension components).
- The research component of the National Integrated Food Safety Initiative focuses on applied food safety research.
- The education component focuses on education and training in a formal classroom setting, which may include elementary, secondary, undergraduate, or graduate education.
- The extension component addresses education and training outside of the classroom. Where there is no extension program, outreach activities that deliver science-based and informational education to people in a variety of nonformal settings are appropriate.
The National Integrated Food Safety Initiative addresses, but is not limited to, the following priority issues in food safety:
- Qualitative and Quantitative Risk Assessments
- Control Measures for Foodborne Microbial Pathogens
- Sources and Incidence of Microbial Pathogens
- Antibiotic Resistant Microbial Pathogens
- Improving the Safety of Fresh Fruits and Vegetables
- National Coordination of Integrated Food Safety Programs and Resources
- Food Handler Education and Training for Consumers and Youth
- Food Handler Education for High-risk and Hard-to-reach Audiences
- Food Handler Education for Commercial and Noncommercial Audiences, Including Food Handler Certification Training and Other Train-the-Trainer Programs
- Hazard Analysis and Critical Control Points (HACCP) Model Development, Testing, and Implementation
- Home Food Processing and Preservation
- Integrating Food Safety into Related Agricultural Programs
- Alternative Food Processing Technologies that Improve the Safety of Food
- Food Security