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Anaerobic digestion is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to manage waste or to produce fuels. Much of the fermentation used industrially to produce food and drink products, as well as home fermentation, uses anaerobic digestion.

Anaerobic digestion occurs naturally in some soils and in lake and oceanic basin sediments, where it is usually referred to as "anaerobic activity". This is the source of marsh gas methane as discovered by Alessandro Volta in 1776.

The digestion process begins with bacterial hydrolysis of the input materials. Insoluble organic polymers, such as carbohydrates, are broken down to soluble derivatives that become available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. In Acetogenesis, bacteria convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide amongst other compounds. Finally, methanogens convert these products to methane and carbon dioxide. The methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments.

Anaerobic digestion is used as part of the process to treat biodegradable waste and sewage sludge. As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digesters can also be fed with purpose-grown energy crops, such as maize.

Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas, consisting of methane, carbon dioxide, and traces of other 'contaminant' gases. This biogas can be used directly as fuel, in combined heat and power gas engines or upgraded to natural gas-quality biomethane. The nutrient-rich digestate also produced can be used as fertilizer.

With the re-use of waste as a resource and new technological approaches that have lowered capital costs, anaerobic digestion has in recent years received increased attention among governments in a number of countries, among these the United Kingdom (2011), Germany, Denmark (2011), and the United States.

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Alzheimer's disease (AD) is a neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in remembering recent events. As the disease advances, symptoms can include problems with language, disorientation (including easily getting lost), mood swings, loss of motivation, self-neglect, and behavioral issues. As a person's condition declines, they often withdraw from family and society. Gradually, bodily functions are lost, ultimately leading to death. Although the speed of progression can vary, the typical life expectancy following diagnosis is three to nine years.

The cause of Alzheimer's disease is poorly understood. There are many environmental and genetic risk factors associated with its development. The strongest genetic risk factor is from an allele of APOE. Other risk factors include a history of head injury, clinical depression, and high blood pressure. The disease process is largely associated with amyloid plaques, neurofibrillary tangles, and loss of neuronal connections in the brain. A probable diagnosis is based on the history of the illness and cognitive testing with medical imaging and blood tests to rule out other possible causes. Initial symptoms are often mistaken for normal aging. Examination of brain tissue is needed for a definite diagnosis, but this can only take place after death. Good nutrition, physical activity, and engaging socially are known to be of benefit generally in aging, and these may help in reducing the risk of cognitive decline and Alzheimer's; in 2019 clinical trials were underway to look at these possibilities. There are no medications or supplements that have been shown to decrease risk.

No treatments stop or reverse its progression, though some may temporarily improve symptoms. Affected people increasingly rely on others for assistance, often placing a burden on the caregiver. The pressures can include social, psychological, physical, and economic elements. Exercise programs may be beneficial with respect to activities of daily living and can potentially improve outcomes. Behavioral problems or psychosis due to dementia are often treated with antipsychotics, but this is not usually recommended, as there is little benefit and an increased risk of early death.

As of 2015, there were approximately 29.8 million people worldwide with AD with about 50 million of all forms of dementia as of 2020. It most often begins in people over 65 years of age, although up to 10 per cent of cases are early-onset affecting those in their 30's to mid 60's. Women get sick more often than men. It affects about 6% of people 65 years and older. In 2015, all forms of dementia resulted in about 1.9 million deaths. The disease is named after German psychiatrist and pathologist Alois Alzheimer, who first described it in 1906. Alzheimer's financial burden on society is large, on par with the costs of cancer and heart disease, with a 2013 study estimating an annual cost of $200 billion (equivalent to $222 billion in 2020) in the US alone.

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Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by inattention, hyperactivity, and impulsivity, which are pervasive, impairing, and otherwise age inappropriate. Some individuals with ADHD also display difficulty regulating emotions, or problems with executive function. For a diagnosis, the symptoms have to be present for more than six months, and cause problems in at least two settings (such as school, home, work, or recreational activities). In children, problems paying attention may result in poor school performance. Additionally, it is associated with other mental disorders and substance use disorders. Although it causes impairment, particularly in modern society, many people with ADHD have sustained attention for tasks they find interesting or rewarding, known as hyperfocus.

The precise cause or causes are unknown in the majority of cases. Genetic factors are estimated to make up about 75% of the risk. Toxins and infections during pregnancy and brain damage may be environmental risks. It does not appear to be related to the style of parenting or discipline. It affects about 5–7% of children when diagnosed via the DSM-IV criteria and 1–2% when diagnosed via the ICD-10 criteria. As of 2019, it was estimated to affect 84.7 million people globally. Rates are similar between countries and differences in rates depend mostly on how it is diagnosed. ADHD is diagnosed approximately two times more often in boys than in girls, although the disorder is often overlooked in girls or only diagnosed in later life because their symptoms often differ from diagnostic criteria. About 30–50% of people diagnosed in childhood continue to have symptoms into adulthood and between 2–5% of adults have the condition. In adults, inner restlessness, rather than hyperactivity, may occur. Adults often develop coping skills which compensate for some or all of their impairments. The condition can be difficult to tell apart from other conditions, as well as from high levels of activity within the range of normal behavior.

ADHD management recommendations vary by country and usually involve some combination of medications, counseling, and lifestyle changes. The British guideline emphasises environmental modifications and education for individuals and carers about ADHD as the first response. If symptoms persist, then parent-training, medication, or psychotherapy (especially cognitive behavioral therapy) can be recommended based on age. Canadian and American guidelines recommend medications and behavioral therapy together, except in preschool-aged children for whom the first-line treatment is behavioral therapy alone. For children and adolescents older than 5, treatment with stimulants is effective for at least 24 months; however, for some, there may be potentially serious side effects.

ADHD, its diagnosis, and its treatment have been considered controversial since the 1970s. The controversies have involved clinicians, teachers, policymakers, parents, and the media. Topics include ADHD's causes and the use of stimulant medications in its treatment. Most healthcare providers accept ADHD as a genuine diagnosis in children and adults, and the debate in the scientific community mainly centers on how it is diagnosed and treated. The condition was officially known as attention deficit disorder (ADD) from 1980 to 1987, and prior to the 1980s, it was known as hyperkinetic reaction of childhood. The medical literature has described symptoms similar to those of ADHD since the 18th century.