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.http://www.doh.wa.gov/notify/other/glossary.htm Glossary of Notifiable Conditions Washington State Department of Health An infection however, is not synonymous with an infectious disease; as an infection may not cause clinical symptoms or impair host function."Infectious disease." McGraw-Hill Encyclopedia of Science and Technology. The McGraw-Hill Companies, Inc., 2005.
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.
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. 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 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.
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).
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.
In some cases, a microorganism and its host live in reasonable harmony. Such is the case for many tropical viruses and the insects, monkeys, or other animals in which they have lived and reproduced for thousands or millions of years. Because the microbes and their hosts have co-evolved together, the hosts have gradually become resistant to the microorganisms. But when a microbe jumps from a long-time animal host to a human being, it may cease being a harmless parasite and—simply because it is new to the human species—become a pathogen. (See infection).
With most new infectious diseases, some human action is involved, changing the environment so that an existing microbe can take up residence in a new niche. Once that happens, a pathogen that had been confined to a remote habitat appears in a new or wider region, or a microbe that had infected only animals suddenly begins causing human disease.
Several human activities have led to the emergence and spread of new diseases:
:Encroachment on wildlife habitats. The construction of new villages and housing developments in rural areas brings people into contact with animals--and the microbes they harbor. :Changes in agriculture. The introduction of new crops attracts new crop pests and the microbes they carry to farming communities, exposing people to unfamiliar diseases. :Destroying rain forests. As tropical countries make use of their rain forests, building roads through forests and clearing areas for settlement or commercial ventures, people encounter insects and other animals harboring unknown microorganisms. :Uncontrolled urbanization. The rapid growth of cities in many developing countries concentrates large numbers of people in crowded areas with poor sanitation, which foster the transmission of contagious diseases. :Modern transport. Ships and other cargo carriers often harbor unintended "passengers," such as insects and rats, that can spread diseases to faraway destinations. :High-speed globe-trotting. With international jet-airplane travel, people infected with a new disease can carry the disease to the far side of the world before their first symptoms appear.
The relationship between virulence and transmission is complex, and has important consequences for the long term evolution of a pathogen. Since it takes time for a microbe and a new host species to co-evolve an emerging pathogen may hit its earliest victims especially hard. It is usually in the first wave of a new disease that death rates are highest. If a disease is rapidly fatal, the host may die before the microbe can get passed along to another host. However, this cost may be overwhelmed by the short term benefit of higher infectiousness if transmission is linked to virulence, as it is for instance in the case of cholera (the explosive diarrhoea aids the bacterium in finding new hosts) or many respiratory infections (sneezing, coughing etc create infectious aerosols).
*H. Krauss, A. Weber, M. Appel, B. Enders, A. v. Graevenitz, H. D. Isenberg, H. G. Schiefer, W. Slenczka, H. Zahner: Zoonoses. Infectious Diseases Transmissible from Animals to Humans. 3rd Edition, 456 pages. ASM Press. American Society for Microbiology, Washington DC., USA. 2003. ISBN 1-55581-236-8