The Ebola virus comprises a group of pathogenic agents that cause severe and deadly hemorrhagic fevers in humans and other primates. During 2014 and 2015, an Ebola virus epidemic in West Africa—predominantly, the countries of Guinea, Liberia, and Sierra Leone—killed more than 11,300 people; it was the worst outbreak of this disease in recorded human history. At that time, the World Health Organization (WHO) declared the Ebola outbreak to be a Public Health Emergency of International Concern (PHEIC), and massive efforts were undertaken to contain the spread of the deadly disease. By early 2016, the affected countries were declared free of Ebola virus transmission, and the epidemic was considered to have ended. However, since then, isolated outbreaks have occurred sporadically, mostly as a result of the virus persisting in survivors after recovery. Most troublesome, though, the Ebola virus was detected in August 2018 in the Democratic Republic of the Congo (DRC) in Central Africa and has since expanded dramatically. It is now the second-largest Ebola outbreak on record. As of June 2019, more than 2000 Ebola cases have been detected, with more than 1350 resultant deaths. See also: Africa; Ebola virus; Ebola virus outbreak in 2014–2015; Exotic viral diseases; Infectious disease; Virus

The benefits of outdoor physical activity (exercise) outweigh the potential harm caused by air pollution except in the most highly polluted cities, according to epidemiological studies reported in the journal Preventive Medicine (February 2016). Exercise provides a number of health benefits, including reduced risks of cardiovascular disease, type 2 diabetes, and some cancers. Cycling and walking, for example, offer not just health benefits but also environmental advantages as pollution-free means of transportation. See also: Air pollution; Cancer (medicine); Epidemiology; Heart disorders; Sports medicine; Type 2 diabetes

The Asian longhorned tick (Haemaphysalis longicornis), also known as the bush tick, is a troublesome parasitic species and is an important vector of disease agents. (Note that vectors are capable of biologically transferring a pathogen from one organism to another.) It also is becoming a problematic invasive species. Specifically, it is invading areas located far from its native region in East and Central Asia and is spreading within the United States. In particular, this arachnid is remarkable because female members can reproduce asexually without mating. Thus, scientists fear that the species will spread rapidly in locations where it has already invaded, thereby transmitting numerous diseases that are potentially harmful to humans and other animals. See also: Acari; Disease; Invasive species; Ixodides; Parasitology; Pathogen; Tick virus diseases; Zoonoses

Recycling or reuse is in most instances environmentally preferable to disposal. Reuse of coal tar as an asphalt-pavement sealant for driveways and parking lots, however, has turned out to be an exception to this rule of thumb. See also: Asphalt and asphaltite; Pavement

Bats are members of the order Chiroptera, which is the second-largest order of living mammals. The geographic distribution of these flying mammals is tremendous as well, ranging from the limit of trees in the Northern Hemisphere to the southern tips of Africa, South America, and New Zealand. Within such a wide distribution, bats frequently encounter human populations. However, close contact between bats and humans is problematic with regard to disease ecology and epidemiology because bats are natural reservoirs (primary hosts) or intermediate hosts for numerous zoonotic pathogens—that is, infectious disease agents that are transmitted from animals to humans. Specifically, bats harbor more than 60 pathogenic viruses that can infect humans, including Ebola, Marburg, Nipah, Hendra, and rabies viruses. Bats also harbor various coronaviruses, including those responsible for severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and coronavirus disease 2019 (COVID-19). See also: Chiroptera; Coronavirus; Disease ecology; Epidemiology; Exotic viral diseases; Infectious disease; Mammalia; Pathogen; Rabies; Virus; Zoonoses

Mosquito repellents are natural and synthetic chemical compounds that are applied to skin, clothing, and sometimes home furnishings such as bed nets. For years, they have been an important defense against mosquito-borne diseases, such as Dengue fever, malaria, and West Nile virus, and most recently Zika virus. See also: Diptera; Insect diseases; Insecticide; Malaria; Mosquito: vector of disease; West Nile virus; Zika virus disease

The novel coronavirus SARS-CoV-2 that emerged in central China in late 2019 and subsequently spread across the world in 2020 led to the COVID-19 pandemic. The emergence of a previously unknown pathogen—such as the novel coronavirus—that could affect much of the world's population has long been anticipated by many epidemiologists, who have been especially concerned about potential consequences of human-influenced (anthropogenic) global climate change as it relates to human health and disease. In fact, as a result of ecological changes attributed to anthropogenic global warming, including altered landscapes and ecosystems, modified predator-prey relationships, habitat destruction, deforestation, and rising land and ocean temperatures, numerous infectious diseases have spread into new geographic regions in recent years, leading to more infections in animal populations. These newly infected animals are interacting with humans to a greater degree than ever before, creating more opportunities for zoonotic disease transmission (that is, transmission from animals to humans). See also: Deforestation; Ecosystem; Epidemiology; Global climate change; Global warming; Infectious disease; Novel coronavirus is declared a global pandemic; Predator-prey interactions; Virus; Zoogeography; Zoonoses

A newly identified coronavirus, provisionally termed 2019-nCoV (an acronym for 2019 novel coronavirus) and subsequently named "severe acute respiratory syndrome coronavirus 2" (SARS-CoV-2), emerged in central China in November 2019. The disease caused by this virus, called COVID-19 (an acronym for "coronavirus disease 2019"), is currently a pandemic (worldwide epidemic) that has sickened and killed millions of people around the world. The infectious virus initially affected individuals in the city of Wuhan, predominantly those working at or frequenting certain animal markets. At first, the virus was thought to spread from animals (possibly bats or pangolins) to people. However, affected individuals who did not have exposure to any animal markets were subsequently identified, indicating that person-to-person transmission was occurring. In addition, studies indicated that the virus remains stable for several hours to days in aerosols and on surfaces; thus, individuals can acquire the virus either via the air or after touching contaminated objects. The many initial unknowns regarding this novel coronavirus and the rapidity with which it is spread prompted the World Health Organization (WHO) to declare COVID-19 a global health emergency on January 30, 2020. On March 11, 2020, the WHO officially announced that COVID-19 was a pandemic. See also: Animal virus; Coronavirus; Disease ecology; Epidemic; Epidemiology; Infectious disease; Virus; Virus classification

There has been substantial progress in the fight against certain drug-resistant organisms that cause serious, often fatal, diseases. However, two drug-resistant microbes—specifically, Candida auris (a fungal yeast) and Acinetobacter (a genus of bacteria)—are responsible for a growing number of drug-resistant infections worldwide. These organisms pose such a potential danger to the health of affected individuals that the Centers for Disease Control and Prevention (CDC) in the United States has added the microbes to its list of pathogenic organisms considered to be urgent public-health threats. [The other pathogens on the CDC's list are Clostridioides difficile (a bacterium formerly known as Clostridium difficile), Neisseria gonorrhoeae (the causative agent of gonorrhea), and various organisms belonging to the Enterobacteriaceae bacterial family.] See also: Bacteria; Candida; Clostridium; Drug resistance; Fungi; Infectious disease; Medical bacteriology; Medical mycology; Microbiology; Pathogen; Public health; Yeast

Tuberculosis (TB) is a chronic infectious disease of humans (as well as animals) primarily involving the lungs. It is caused by the tubercle bacillus Mycobacterium tuberculosis and is transmitted by pathogenic airborne droplets. Approximately 5–10% of individuals infected with the TB bacterium progress to develop the active disease, and these individuals must be treated with a combination of antibiotics, taken for several months, for effective elimination of the disease. However, treatment failure or relapse is on the rise as a result of drug-resistant strains of the M. tuberculosis organism. See also: Antibiotic; Antibiotic resistance; Antimicrobial resistance; Bacteria; Bacteriology; Clinical microbiology; Drug resistance; Lung; Medical bacteriology; Microbiology; Mycobacterial diseases; Pathogen; Tuberculosis