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

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

Scientists have suggested a possible connection between ancient Neandertals (Homo neanderthalensis) and the severity of disease in patients affected by COVID-19. In short, certain genes acquired from Neandertals during instances of interbreeding with ancestors of modern humans (Homo sapiens) approximately 60,000 years ago may be involved in the susceptibility of individuals to severe disease from the virus that causes COVID-19. Specifically, researchers have identified a region on human chromosome 3 that possibly influences whether a person infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) will become seriously ill and require artificial ventilation. This chromosomal region is almost identical to a segment found in the DNA of Neandertals from Croatia and Siberia, and present-day carriers of the Neandertal-derived gene variant on human chromosome 3 have an increased likelihood of developing life-threatening forms of COVID-19. See also: Chromosome; Coronavirus; Deoxyribonucleic acid (DNA); Gene; Neandertal DNA; Neandertals; Novel coronavirus is declared a global pandemic

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

Honeybees (Apis mellifera), the predominant pollinators of economically important plants and crops, have been facing sharp population declines around the world since the 1980s. Recently, the die-off of honeybees has been dramatic, with beekeepers in the United States losing more than 42% of their colonies between April 2014 and April 2015, after a 5-year period in which the annual bee losses averaged more than 30%. This troublesome statistic is made even more alarming by the fact that the number of bee deaths in summer exceeded those in winter. Because summer is the most productive and vigorous time for bees, any disruption of their health during the summer season is severely detrimental. Studies have linked the deaths of the pollinators to numerous and varied stressors, including viral pathogens, parasitic pests (such as Varroa mites), habitat loss, climate change, inadequate nutrition, compromised immune systems (possibly as the result of colony collapse disorder), and exposure to pesticides and insecticides (in particular, neonicotinoids). However, no individual stressor has yet been singled out in the scientific consensus as a primary culprit, so it is likely that a combination of these stressors, and possibly others, is to blame. See also: Beekeeping; Colony collapse disorder; Herbicide; Honeybee genome; Hymenoptera; Immunosuppression; Insect diseases; Insecticide; Pathology; Pesticide; Pollen; Pollination; Social insects; Virus

Eastern equine encephalitis is a rare infectious viral disease that primarily affects humans and equines (horses, mules, donkeys, and zebras). It is caused by a mosquito-borne virus (arbovirus) that is native to the eastern half of the United States, particularly in coastal regions. The virus also has been detected elsewhere in North America, as well as in South America and the Caribbean. Categorized as a zoonotic disease (that is, a disease transmitted from animals to humans), infected mosquitoes are the transmitting agents (vectors) responsible for spreading the virus that causes it. The virus predominantly affects birds—including chickens—which remain asymptomatic of the disease. However, Eastern equine encephalitis can be lethal when contracted by humans and horses because the virus can invade the central nervous system and lead to brain swelling and damage, which often results in death. As such, investigators are troubled by a recent dramatic increase in the detection of Eastern equine encephalitis virus in Florida. See also: Animal virus; Arboviral encephalitides; Economic entomology; Encephalitis (arboviral); Exotic viral diseases; Infectious disease; Mosquito; Virus; Zoonoses

Ebola virus disease is one of the most lethal and feared diseases in the modern world even though the total number of people harmed by it throughout history is, fortunately, far smaller than for better known scourges such as malaria and influenza. Among individuals who contract this disease, the fatality rate averages approximately 50 percent, although it can reach as high as 90 percent. Outbreaks of this highly infectious disease are therefore of great concern to infected communities and medical personnel, especially when the populations at risk are large. The disease arises from infection with one of the Ebola virus strains, which comprise a group of exotic viral agents that cause a severe hemorrhagic fever in humans and other primates. The virus is spread through human-to-human transmission and requires direct contact with the bodily fluids or blood of an infected person. It also can be transmitted via surfaces and objects that have been contaminated with these fluids. Airborne transmission does not occur. (Other types of hemorrhagic diseases caused by RNA viruses include dengue fever, hantavirus pulmonary syndrome, Lassa fever, and yellow fever, many of which are transmitted by insects and rodents rather than by direct contact.) See also: Ebola virus; Epidemiology; Exotic viral diseases; Hantavirus outbreak; Infectious disease; Virus; Virus classification

A serious outbreak of equine herpesvirus type 1 (EHV-1), an infectious viral disease, is currently spreading among horse populations in Europe. This outbreak was first detected in Valencia, Spain, in February 2021 during a month-long equestrian competition. Multiple dozens of horses contracted an initially unknown disease, with up to 20 horses being treated daily for a variety of symptoms. Some of these symptoms were unusually severe. Many horses showed drastic behavioral changes, pointing to neurological damage, whereas other animals experienced respiratory distress, blood clots, or general malaise. Before the origin of the disease could be identified and before the vast majority of horses could be quarantined, the Spanish competition ended and more than 600 horses returned to their home stables located throughout Europe. Once the pathogen was identified as EHV-1, the Fédération Équestre Internationale (International Federation for Equestrian Sports) postponed or cancelled all European equine events through May 2021. Concerns over transmission of this virus may extend the pause in equine events throughout Europe beyond this spring. See also: Agricultural science (animal); Animal virus; Herpes; Horse production; Infectious disease; Virus

The emergent discipline of evolutionary epidemiology recognizes that pathogens (disease-causing organisms) are biological entities that can evolve quickly to take advantage of ecological or environmental changes that promote their transmission. Virulence, a measure of a microorganism's ability to infect and sicken a host, also evolves, which allows previously mild diseases to become life-threatening ones, or vice versa. Pathogens and virulence evolve because the life cycle of pathogens is typically very short compared to that of their hosts. The majority of bacteria and viruses replicate numerous times daily, enabling natural selection to favor variants that thrive against the relatively stable, more slowly adapting host. In addition, virulence varies not only from pathogen to pathogen, but also from strain to strain. For example, Escherichia coli can cause fatal foodborne illnesses, yet harmless strains of this bacterium inhabit the intestinal tract of humans. Thus, evolutionary epidemiologists seek to explain the factors contributing to the evolution of pathogens and virulence. See also: Bacteria; Disease; Epidemiology; Escherichia; Escherichia coli outbreaks; Food safety and foodborne illness; Infectious disease; Organic evolution; Pathogen; Virulence; Virus