throughout the Life Span (Elsevier Mosby, 2006); Healthy. People ... one person to another or between animals and humans. (animal to animal ... ternal region of the body of the host. ... The infectious agent is carried by healthy hosts called.
Infectious Diseases (General)
and toddlers become mobile, health risks increase. Therefore, Healthy People 2010 is also working to reduce the risk of drowning deaths among children under the age of 4 to no more than 0.9 per 100,000, increase the use of automobile child restraints for this age group to 100 percent, reduce antibiotic courses for ear infections to 88 courses per 100 children under the age of 5, and reduce lead blood levels in all children under the age of 5. SEE ALSO: AIDS; Autism; Birth Defects; Disease Preven-
tion; Gastroenterology; Infant and Toddler Development; Otology; Pregnancy; Preventive Care; Respiratory Diseases (General); Sudden Infant Death Syndrome (SIDS); United Nations Children’s Fund (UNICEF); World Health Organization (WHO). BIBLIOGRAPHY. Michael S. Clement, Children at Health
Risk (Blackwell, 2003); Andrew Curtis and Michael Leitner, Geographic Information Systems and Public Health: Eliminating Perinatal Disparity (IRM Press, 2006); Carole Lium Edelman and Carol Lynn Mandle, Health Promotion throughout the Life Span (Elsevier Mosby, 2006); Healthy People, http://www.healthypeople.gov/ (cited July 2007); Jenifer, Swanson, Infant and Toddler Health Source Book: Basic Consumer Health Information (Omnigraphics, 2000); H.,B. Valman and R.M. Thomas, ABC of the First Year (BMJ Books, 2002); UNICEF, “One in Four Infants Still at Risk from Vaccine Preventable Diseases” http://www.unicef. org/media/media_28400.html (cited cited July 2007); James Wynbrandt and Mark D. Ludman, The Encyclopedia of Genetic Disorders and Birth Defects (Facts on File, 2000). Elizabeth R. Purdy, Ph.D. Independent Scholar
Infectious Diseases (General) By definition, those diseases that are transmitted from one person to another or between animals and humans (animal to animal, animal to human, human to human, human to animal), implying the transfer of a microorganisms or/and an infectious pathogen agent, are infectious diseases. Infectious agents are living (or at least nucleic acid encoding or proteins) units that must invade the insect host in order to initiate an infection.
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Infectious diseases could be caused by bacteria, fungi, viruses, parasites (protozoan and helminthes) and prions. Ectoparasites produce infestation, a different term to express the presence of a pathogen agent on the external region of the body of the host. Pathogens can be spread by many routes other than direct contact, including through water, food, air, and bodily fluids—blood, semen, saliva, and so on. Many diseases may be transferred by vectors—animals (usually insects) that carry microorganisms (mostly viruses and parasites). Vectors may spread a disease either by mechanical or by biological means (the organisms require a vector to reach specific stages of its life cycle, particularly the infectious stage for the intermediary, accidental and definitive hosts). Mechanical transmission occurs, when flies transfer the germs for typhoid fever from the feces (stool) of infected people to food eaten by healthy people. Biological transmission takes place when an insect bites a person and takes infected blood into its own system. Once inside the insect’s gut, the disease-causing organisms may reproduce, increasing the number of microorganisms that can be transmitted to the next victim. This is how the Anopheles spp. or Aedes spp. mosquito vector, for instance, transfers malaria or dengue, respectively. Additionally, the advance in new areas of activity has let the emergence of new infectious diseases as well new pathogens behavior, and the modification of established methods of transmission. In the medical management of patients, particularly in hospitals, healthcare-associated infections are also a raising concern worldwide (nosocomial infections). Historically, the record of human suffering and death caused by smallpox, cholera, typhus, dysentery, malaria, and so forth establishes the eminence of the infectious diseases. Despite the outstanding successes in control afforded by improved sanitation, immunization, and antimicrobial therapy, the infectious diseases continue to be a common and significant problem of modern medicine. The most common disease of humankind, the common cold, is an infectious disease, as is the feared modern disease AIDS. Some chronic neurological diseases that were thought formerly to be degenerative diseases have proven to be infectious. Day by day, news scientific findings have revealed the role of the infectious agents in chronic diseases such as asthma, multiple sclerosis, some chronic cardiovascular diseases, Whipple disease, gastric diseases, diabetes, and many types of cancer, among others. There is little
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Infectious Diseases (General)
doubt that the future will continue to reveal the infectious diseases as major medical problems. The infectious agent is carried by healthy hosts called reservoirs (e.g., water birds for the flu virus). It is transmitted to the target organism either directly or through carrier organisms. For example, the protozoa responsible for malaria live in mosquitoes, carriers that transmit them through their bites to humans, who develop the disease. The worldwide emergence of diseases once restricted to a particular region is caused by changes in lifestyle and human modification of the environment due to high demographic growth (clearing and deforestation, contact with wild fauna, overcrowding in megalopolises, tourism, immigration, etc.). Since an epidemiological point of view, worldwide, bacterial and viral infectious diseases represent and call the major attention of healthcare systems, in part due to the economical burden for them, related to the medical management of such infections, such antibiotics for bacterial infections. Treating viral illnesses or noninfective causes of inflammation with antibiotics is ineffective, however, and contributes to the development of antibiotic resistance, toxicity and allergic reactions, leading to increasing medical costs. For these reasons in the field of infectious diseases different markers related to distinguish the general etiology of the disease have been aimed and developed. Markers such as the leukocyte and neutrophil counts, serum C-reactive protein (CRP) level and erythrocyte sedimentation rate (ESR), which have relatively poor sensitivity and specificity. But other news, such as procalcitonin, inflammatory mediators such as G-CSF, TNF-a, IL-1b, IL-6, and IL-8, the triggering receptors expressed on myeloid cells (TREM), the neutrophil FcgRI (CD64), the phagocyte complement receptors and the product of platelet and neutrophil counts (PN product), among others, which have been showing relatively good sensitivity and specificity. There are a shifting pattern in the profile of infectious diseases among the countries, developed and developing countries. The majority of developed countries have undergone a prototypical epidemiologic and demographic transition. Host–infectious agent relations have evolved over centuries, but these transitions are largely attributed to a decrease in the burden and mortality from infectious diseases. As a result, life expectancy has radically changed over the past century
with a significant increase in longevity. Although we welcome these improvements in health status, we never imagined that prolongation of life due to fewer infectious diseases would have disadvantages in future generations. It has become evident that inflammatory disorders, including both autoimmune and allergic diseases, are increasing in prevalence to epidemic proportions, particularly in more affluent, industrialized countries over the past 40 years. At the same time, the rate of allergic diseases in the developing world has not changed over the same period of time. There is current scientific evidence that a major player in explaining this tendency could be partially explained by decreased incidence of infectious diseases. In the case of developing countries the situation is different. The world is marked by extremes of economic inequality, across and within countries in which poverty is a common denominator. With incomes in resource-rich countries exceeding thousands of dollars, 20 percent of the world’s population nonetheless survives on less than $1 a day. Inevitably, the economic health of a country both affects and is affected by its people’s health and life expectancy. It has been argued that the health effects of inequalities with respect to income and assets impose a major burden on the poor, which reduces the competitiveness of societies and has an effect on the global marketplace. Therefore, it is in the world’s best interest to ensure the health of all its people, because human health is the foundation of economic growth and development. Many infectious diseases previously seen only in resource-constrained settings can be currently diagnosed anywhere in the globe due to increased travel, immunosuppression, HIV/AIDS; organ transplantation and blood transfusion. SEE ALSO: Environmental Health; Epidemiology; Immu-
nology; Medical Entomology.
BIBLIOGRAPHY. Gordon Cook and Alimuddin Zulma,
Manson’s Tropical Diseases (Saunders, 2003); Paulo Sergio Lucas Da Silva, et al., “The Product of Platelet and Neutrophil Counts (PN Product) at Presentation as a Predictor of Outcome in Children with Meningococcal Disease,” Annals of Tropical Paediatrics (v.27/1, 2007); Erick Folch, et al., “Infectious Diseases, Non-Zero-Sum Thinking, and the Developing World,” American Journal of Medical Sciences (v.326/2, 2003); Carlos Franco-Paredes, Ildefonso
Infertility
Tellez, and Carlos del Rio, “Inverse Relationship between Decreased Infectious Diseases and Increased Inflammatory Disorder Occurrence: the Price to Pay,” Archives of Medical Research (v.35/3, 2004); Carlos Franco-Paredes, et al., “Cardiac Manifestations of Parasitic Infections Part 3: Pericardial and Miscellaneous Cardiopulmonary Manifestations,” Clinical Cardiology (v.30/6, 2007); Jari Nuutila, and Esa-Matti Lilius, “Distinction between Bacterial and Viral Infections,” Current Opinion in Infectious Diseases (v.20/3, 2007). Alfonso J. Rodriguez-Morales, M.D., M.Sc. Universidad de Los Andes, Venezuela Carlos Franco-Paredes, M.D., M.P.H. Emory University
Infertility The clinical definition of infertility is one year of having normal sexual activity without pregnancy, without using contraceptive methods. According to the World Health Organization, infertility affects one out of 10 couples at some point in their lives, with 80 million people affected worldwide. Males contribute to between 40 to 50 percent of infertility cases. While normal male spermatogenesis means men are potentially fertile all the time and throughout their lives, women have a small window each month of approximately six days, ending with ovulation when they are fertile and a limited fertile time between the onset of menses and the cessation of menstruation at menopause. To maximize the chance of getting pregnant, both women and men should understand how the reproductive system works to ensure they are timing sexual activity during a fertile time. The causes of infertility can be on the part of either the man or the woman and vary from blockage in the sexual organs, to being secondary from other illnesses or biologic indicators, to having psychological characteristics, or being caused by environmental exposure to toxic substances. Similarities of infertility issues can be analogous between the genders in the case of hormonal considerations. Not producing or releasing enough sex hormones can lead to anovulation or lack of spermogenesis. The hypothalamus releases gonadotropin releasing hor-
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mone (GnRH), to signal the pituitary to release luteinizing hormone (LH) and follicle stimulating hormone (FSH), which in turn regulates the gonads. A feedback loop of steroid hormones in the circulating blood turns off or stimulates hypothalamic activity. Male contributing factors A reproductive health exam with a specialist will take into account common general factors affecting male fertility, including stress, obesity (estrogen production by adipose tissue blocks GnRH release), genetic disorders, cancer, and tobacco, drug, or anabolic steroid use. Specific factors affecting male fertility include undescended testicles, erectile dysfunction, premature ejaculation, excessive heat (increased temperature impede sperm production), blockage in spermatic cord, and viability of sperm. Average seminal fluid has a neutral pH (approximately 7.2) and contains nutrients for the sperm to use for energy. On average, 100 to 300 million sperm are produced each day. Fertile ejaculate contains adequate volume and concentration of sperm (100 to 400 million sperm) with enough sperm of normal morphology or shape, and the motility or ability to move from the vagina to the fallopian tubes. Problems with morphology or motility are apparent with the occurrence of sperm with pinpointed head, tapered heads, crooked heads, two heads, and coiled or kinked tails. Primary hypogonadism is defined as testicular failure, and secondary hypogonadism is defined as defective secretion of gonadotropins from the hypothalamus and/or pituitary. These hormones target three specialized cells for reproduction—the spermatogonia in the seminiferous tubules, the Leydig cells in the connecting tissue between the coiled seminiferous tubules, and the Sertoli cells forming the basement member of the seminiferous tubules. They provide the environment necessary for germ cell differentiation and maturation. Female contributing factors As with male reproductive health, general factors affect female fertility, including age (women in their late 30s are less fertile than those in their 20s), excessive exercise, stress, weight (either loss or gain), chronic diseases (lupus, polycystic ovary syndrome, arthritis, hypertension, asthma, cancer), and tobacco or drug use. Specific factors affecting female fertil-
