A viral disease of humans caused by the human immunodeficiency virus (HIV), which attacks and compromises the body's immune system. Individuals infected with the human immunodeficiency virus (HIV) proceed through a spectrum of stages that ultimately lead to the critical end point, acquired immune deficiency syndrome (AIDS). The AIDS disease is characterized by a profound progressive irreversible depletion of T-helper-inducer lymphocytes (CD4+ lymphocytes; Fig. 1), which leads to the onset of multiple and recurrent opportunistic infections by other viruses, fungi, bacteria, and protozoa, as well as various tumors (Kaposi's sarcoma and lymphomas). HIV infection is transmitted by sexual intercourse (vaginal, anal, or oral), by blood and blood products, and perinatally from an infected mother to a child (prepartum, intrapartum, and postpartum via breast milk). See also: Human immunodeficiency virus (HIV); Opportunistic infections

An acute, infectious worldwide zoonotic disease (transmissible to humans from other animals) caused by the spore-forming bacterium Bacillus anthracis to which most animals, especially grazing herbivores, are susceptible. Anthrax is a serious, though rare, infectious disease caused by a rod-shaped bacterium, Bacillus anthracis (Fig. 1). In natural conditions, anthrax infections in humans are predominantly cutaneous and usually result from contact with infected animals or contaminated animal products, such as hides or wool. The awareness of the use of Bacillus anthracis as a bioterrorist weapon has considerably increased. Anthrax is endemic as a zoonosis in many areas of Africa, Asia, and the Americas, where spores can lie dormant in the soil for many years and commonly affect grazing animals, including sheep, cattle, and goats. See also: Agricultural science (animal); Anthrax bacillus and the immune response; Bioterrorism; Infectious disease; Medical bacteriology; Zoonoses

The capability that a bacterium acquires to counteract an inhibitory chemical molecule or compound that was formerly effective in killing it or preventing its growth. Antibiotic resistance in bacteria (Fig. 1) is a hot topic in the media and in scientific and health care settings worldwide. Antibiotic-resistant bacteria pose a catastrophic threat to people in every country in the world. It is estimated that more than 2.8 million people in the United States become infected by resistant bacteria each year and that, worldwide, around five million people might die as a consequence of an infection by antimicrobial resistant (AMR) bacteria per year. The CDC indicates that, in the United States, at least 23,000 people die per year as a direct result of AMR infections, with more individuals dying because of complications that occur as a result of these infections. The CDC keeps an pdated list of infections considered as urgent threats to public health and, depending on the year, at the top of the list there have been organisms like carbapenem-resistant Enterobacteriaceae (CRE), drug-resistant gonorrhea, carbapenem-resistant Acinetobacter (CRAB), Clostridioides difficile (formerly, Clostridium difficile), which is linked to serious diarrheal illnesses associated with antibiotic use, and methicillin-resistant Staphylococcus aureus (MRSA). The current status of antibiotic resistance worldwide needs to be investigated meticulously, and ways must be found to halt its progression. See also: Antibiotic; Antimicrobial agents; Methicillin-resistant Staphylococcus aureus (MRSA); Bacteria; Bacteriology; Drug resistance; Infection; Medical bacteriology; Microbiology; Public health

Several brain-inflammation diseases, including Saint Louis encephalitis, Japanese encephalitis, and various forms of equine encephalitis, caused by arthropod-borne viruses (arboviruses). In their most severe human forms, the diseases collectively termed arboviral encephalitides (see table) invade the central nervous system and produce brain damage, with mental confusion, convulsions, and coma; death or serious aftereffects are frequent in severe cases. Inapparent infections are common. See also: Arthropoda; Disease ecology; Exotic viral diseases; Zoonoses

A highly contagious intestinal disease, also known as shigellosis, caused by rod-shaped bacteria of the genus Shigella. Bacillary dysentery, which also is referred to as shigellosis, is a significant intestinal infection. Rod-shaped bacteria of the genus Shigella are the causative agents of this disease (see illustration). Transmission occurs by the fecal–oral route. The global disease burden is estimated as 80–165 million cases annually. At least 600,000 deaths per year are attributed to bacillary dysentery. The disease is particularly prevalent in children living in less industrialized countries. Occasionally, common-source outbreaks occur in industrialized countries, usually as a result of contaminated food. The two most common species involved in bacillary dysentery are S. sonnei and S. flexneri. Epidemics of S. dysenteriae also can occur in equatorial regions, and these outbreaks usually involve adults as well as children. See also: Bacteria; Bacteriology; Food microbiology; Food poisoning; Food science; Foodborne disease; Infectious disease; Medical bacteriology; Public health

A parasitic disease caused by a roundworm typically found in raccoons that is transmissible to humans, other mammals, and birds. Baylisascaris procyonis is a parasitic roundworm of raccoons (Procyon lotor) and belongs to the order Ascaridida in the phylum Nematoda (Nemata). This large parasite, which is commonly termed raccoon roundworm, is responsible for the disease baylisascariasis in animals and humans. The disease is manifested by severe or fatal visceral larval migrans, ocular larval migrans, and neurological disease (neurological larval migrans) in both animals and humans. Larval migrans is defined as the prolonged migration and persistence of helminth larvae in the organs and tissues of animals or humans. This infection is considered by the Centers for Disease Control and Prevention as an emerging zoonosis (a disease of animals that is transmissible to humans) in the United States. It was first reported in the United States in 1931 from a raccoon in the New York Zoological Park (now known as the Bronx Zoo). The infection appears to be common in raccoons across the United States, including California, the Midwest, the Northeast region, the Middle Atlantic region, and the mountainous areas of the southeastern states. The first human case was reported in 1984 in a 10-month-old child in Pennsylvania. At least 16 severe cases (including 6 fatalities) have been reported and well documented in humans. The prevalence of subclinical cases is unknown. See also: Ascaridida; Infection; Nemata (Nematoda); Parasitology; Raccoon; Zoonoses

Identifying exposure sources, routes, frequency, duration, and magnitude using biomarker data combined with other approaches. The goal of environmental health science is to understand the interplay between the environment and humans to evaluate the effects of human activities on the public health and environment and, conversely, to evaluate the effects of various aspects of the environment on human health. When investigating the effects that exposures to chemicals have on human health, the major challenge lies in establishing the causal relationship between the magnitude of exposure to these chemicals and the incidence of adverse outcomes (such as cancer and irritation) at various biological endpoints. This causal relationship can be established only when all elements on the source–exposure–dose–effect continuum are linked (Fig. 1).

A form of terrorism in which biological agents (viruses, bacteria, fungi, or toxins) are intentionally released in order to cause death or disease in humans, animals, and/or plants; also referred to as biowarfare or germ warfare, particularly when practiced by military forces. Terrorism is defined by the U.S. government as the “unlawful use of force and violence against persons or property to intimidate or coerce a government, the civilian population, or any segment thereof, in furtherance of political or social objectives.” The term bioterrorism does not imply that only biological agents are used in a particular attack. For example, terrorists can also utilize traditional weapons (for example, guns), chemical agents, or nuclear bombs. Although a biological agent may injure or kill humans, animals, or plants, the main aim of terrorists is to further their social and political goals by making their civilian targets feel that their government cannot protect them (Fig. 1). There are many biological agents found in nature; however, they also can be modified by the terrorist to make them even more dangerous. Some of these agents can be transmitted from person to person, and the infection may take hours or days to become apparent. A topic of growing concern is agroterrorism—the use of microbes to terrorize humans by destroying the food supply. See also: Bacteria; Bacteriology; Biotechnology; Fungi; Fungi and fungal toxins as weapons; Toxin; Virus

A rare inflammatory lung disease affecting the small airways (bronchioles) and air sacs (alveoli) of the respiratory system. Bronchiolitis obliterans organizing pneumonia (BOOP) was first described in 1985 by Gary Epler as a disease distinct from other disorders of the lungs; it is also known as Epler's pneumonia and cryptogenic organizing pneumonia (COP). In general, BOOP is characterized by the formation of connective tissue masses in the lungs, filling the respiratory bronchioles and making breathing difficult and painful. BOOP presents clinically with initial flulike symptoms, followed by progressive dyspnea (shortness of breath), cough, fever, and weight loss. Crackling sounds can be heard within the chest using a stethoscope. X-ray imagery shows bilateral patchy infiltrates in the alveoli of the lungs. Computerized tomography (CT) shows bilateral areas of lung consolidation (see illustration). This consolidation appears with a ground-glass-like haziness that is characteristic for the disease. See also: Computerized tomography; Inflammation; Lung; Pneumonia; Respiratory system; Respiratory system disorders

An infectious disease of various animals and humans caused by Brucella bacteria. Brucellosis (also known as Malta fever, Mediterranean fever, and undulant fever) is a significant disease of many animals. It affects numerous mammals, primarily cattle, dogs, goats, sheep, and pigs. Humans also can contract the disease zoonotically (that is, the disease is transmitted from animals to humans), although it is less prevalent among humans compared to the aforementioned animals. Each bacterial species of Brucella tends to preferentially infect a particular animal, but several types can infect humans. Brucella melitensis (which preferentially infects goats and sheep), B. suis (which infects pigs), and B. abortus (which infects cattle) are the most common causes of human brucellosis. Brucella melitensis is the most virulent type for humans, followed by B. suis and B. abortus. Brucella canis (see illustration) and B. ovis, which infect dogs and sheep, respectively, rarely infect humans. Although brucellosis is found all over the world, the disease has been eradicated in many countries. Brucella bacteria (brucellae) are small, Gram-negative coccobacilli. They are defined as facultative intracellular parasites because they are able to replicate within specialized cells of the host. See also: Bacteria; Bacteriology; Epidemiology; Infectious disease; Medical bacteriology; Microbiology; Parasitology; Zoonoses