The natural history of disease refers to the progress of the disease process in an individual over time and in the absence of intervention (Figure 1.1). Knowledge of the natural history of a disease helps us to understand the effects and mechanism of actions, potential interventions, and the different levels of the prevention of disease.
Natural history of disease Source: based on CDC (2002). The disease process begins with exposure to, or completion of, a sufficient cause of the disease. Without an appropriate intervention, the process ends with recovery, disability or death. For example, exposure to the measles virus in a susceptible individual initiates the stage of subclinical disease. The onset of fever on about the 10th day (range 7-18 days) after exposure marks the beginning of clinical disease. The disease, however, is usually diagnosed around the 14th day when the typical rashes appear and then the disease proceeds to recovery, to complications such as pneumonia, or to death, depending on host and other factors (Sulkowski Et Al., 2007).
Many diseases have a typical natural history, but the time frame and manifestations of disease may vary between individuals due to the presence of host factors (e.g. immunity and age) and other determinants of the disease. Many factors may affect the progress of a disease in an individual and the likely outcome. The estimation of an individual's outcome, taking into account the natural history of disease and other risk factors, is known as their prognosis. The course of a disease may also be modified at any point in the progression by preventive and therapeutic measures. The subclinical stage following exposure is usually called the incubation period (for infectious diseases) or the latency period (for chronic diseases).
In a relatively short time period, a great deal of knowledge was accumulated regarding the biology, pathogenesis, and natural history of infection with HIV. Application of serological testing revealed a wide range of clinical manifestations associated with HIV, including asymptomatic carriage for several years, nonspecific symptoms (often called AIDS-related complex) such as fever, diarrhea, weight loss, and generalized lymphadenopathy-increased susceptibility to certain opportunistic pathogens, increased severity of infection with certain conventional pathogens, development of malignancies including KS, primary lymphoma of the brain, other non-Hodgkin's lymphomas, and progressive encephalopathy (HIV dementia) (Wilkinson, 1996).
As the spectrum of illness associated with HIV expanded, the utility of the surveillance definition of AIDS used by the Centers for Disease Control in collecting case reports came under question. The case definition, which had undergone minor revisions and additions between 1982 and 1986, was designed to count only documented infections or malignancies that indicated severe immunodeficiency and occurred in the absence of known causes of reduced resistance. For each indicator disease, there were specific criteria for establishing a definitive diagnosis. Invasive procedures such as bronchoscopy or biopsy, and sophisticated laboratory analysis were often needed to establish that a patient had "CDC-defined" AIDS. If the specified tests were not done or were negative, the patient was not counted as an official AIDS case. In addition, morbidity and mortality due to AIDS-related complex, wasting syndrome, and encephalopathy were not included. Concern developed that a significant amount of HIV-related disease was being overlooked. In September 1987, a new surveillance definition with extensive changes was published (Bedimo Et Al., 2009) in an attempt to incorporate a broader range of HIV-related disease. Presumptive as well as definitive criteria for the diagnosis of opportunistic infections were listed, and HIV wasting syndrome and HIV dementia were added. A number of new infections and malignancies were added. Laboratory evidence of HIV infection (antibody testing, viral culture, or antigen detection) was also required in most of the cases.
An examination of the clinical spectrum of HIV-related disease and the changes in criteria for CDC-defined AIDS reveals a conceptual paradox for epidemiologists that has resulted from the discovery of AIDS and of HIV. "AIDS" is neither a disease nor a syndrome as traditionally defined. To have "AIDS," one must have some other disease, such as Pneumocystis carinii pneumonia or Kaposi's sarcoma. A person with "AIDS" may have both PCP and KS, as well as some other indicator disease, such as a herpes lesion that is progressive and fails to heal after four weeks. But a person with HIV infection who becomes ill and dies does not have "AIDS" unless evidence for one of these or a number of other conditions is found. Thus, "AIDS" is defined in relationship to other known diseases (Sulkowski Et Al., 2007). The statements "He has AIDS because he has PCP" and "He has PCP because he has AIDS" are both correct. The first describes the diagnosis of AIDS based on evidence for an opportunistic infection; the second describes the development of an opportunistic infection as a result of severe immunodeficiency.
The recognition of AIDS and discovery of HIV represent events that could not have occurred without the advances in virology and immunology that preceded them. In linking two separate disease syndromes (PCP and KS) under a common syndrome, epidemiologists created a conceptual paradox at the same time they removed one of the major obstacles that had prevented viruses from being established as the cause of human chronic diseases. In this instance, the one agent/one disease requirement was abandoned, and the actual complexity of disease etiology was exposed to study.
Web of Causation
"There is, therefore a serious danger that viruses from such closely related groups as the simian primates could show an altered pathogenesis in man, of which malignancy could be a feature" (Weber Et Al., 2006). A desire to understand the origin of things derives from a basic human curiosity and underlies much of anthropological research and theory. Questions related to origin characterize a substantial portion of other scientific endeavors as well. In the case of most infectious diseases, hypotheses regarding their origin are speculative and untestable.
Like HIV-I, the viruses that cause measles and smallpox are highly species specific, and rarely infect or cause disease in other animals. Yet disease theorists have usually hypothesized an animal origin for measles and smallpox. The existence of closely related viruses in domestic animals is used as evidence (Bedimo Et Al., 2009). This type of evidence is equally supportive of the hypothesis that the animals in question acquired their diseases from contact with humans. However, Western scientists try to find the cause of disease in "nature" rather than in humans (Bedimo Et Al., 2009).
For the layperson, when the topic is an infectious agent that produces human suffering and death, the question of origin becomes confused with the idea of responsibility. In a lot of civilizations, epidemics are seen as unnatural happenings caused by a variety of taboo infringements. Yet in modem Western cultures, victim responsibility and presenting disease as reprimand from God are common. All through history, sickness has frequently been held liable on "outsiders," as defined by race, ethnicity, religion, or nationality. In Western cultural concepts, disease is considered unnatural, and the genesis of disease is best placed as far from "people like us" as possible. "Outsiders" may be humans who are different, or better yet, some other animal species (Weber Et Al., 2006).
Speculation concerning the origin of AIDS provides a modem example of these tendencies. An elaborate evolutionary scenario involving green monkeys, Africans, Haitians, and vacationing American homosexuals has been constructed. For various political and economic reasons, aspects of this scenario have become the subject of a bitter controversy (Green, 2009). In the form it has taken in the lay press, this scenario has all of the characteristics of a "just so" story. As related in the December 1985 issue of Discover, the progenitor of HIV-I is found in the African green monkey. People in Central Africa who ate or were bitten by the monkeys acquired the virus, which was then transmitted to Haitians visiting Zaire. In Haiti, vacationing American homosexuals acquired the infection, and brought it back to New York (Green, 2009).
By accepting this paradigmatic view of disease causation, individuals and communities may unwittingly contribute to the depolitization of AIDS transmission. Such a viewpoint assumes that individuals, even impoverished minority groups members, have adequate resources to lead a healthy life and to engage in personal AIDS risk reduction (Weber Et Al., 2006).
The development of the third anti-camping ordinance occurred during the same period that the County began massive budget cuts in response to the largest municipal bankruptcy case in U.S. history. The bankruptcy, which resulted from massive losses emanating from a risky municipal investment pool caused service cutbacks across departments and services, and translated into direct service and monetary support cutbacks for Orange County's low-income residents (Sulkowski Et Al., 2007). Of the $40.2 million in budget reductions approved by the County Board of Supervisors across twenty-seven County departments and programs, 40 per cent or $16.6 million had supported health care, social services, and defense attorneys for the County's low-income residents (Oursler Et Al., 2006). Rent assistance was cancelled, and…