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Mountains are elevated landforms whose slopes and summits stand above surrounding terrain. They occur in many shapes, from sharp rocky peaks to broad rounded ridges. Their form reflects both the processes that raised the land and the processes that continue to wear it down. A mountain landscape is therefore a record of change rather than a collection of permanent shapes.

Some mountain ranges form where large sections of the Earth’s outer shell move toward one another. Other mountains are associated with volcanic activity or the movement of blocks along faults. These different origins can produce different arrangements of rock. Once exposed at the surface, the rock is affected by water, changing temperatures, ice, and gravity. Broken material may remain on a slope or travel into valleys and river systems below.

Conditions often change noticeably with elevation. Air temperature generally decreases upward, while exposure to wind can increase on ridges. The amount and timing of rainfall or snowfall also vary between slopes. A hillside facing the sun may support different vegetation from a shaded hillside nearby. These contrasts create a range of habitats within a relatively small area, even when the entire mountain appears uniform from a distant viewpoint.

Mountain valleys help direct the movement of water and people. Streams collect runoff and carry sediment away from higher ground. Passes provide routes through ridges, while steep slopes can make travel difficult. Settlements, paths, and cultivated fields often occupy the more accessible parts of the landscape. The placement of these features shows how people adapt to local terrain rather than treating every part of a mountain as equally usable.

Looking closely at a mountain reveals many scales of activity. A loose stone may move after rain, a stream may cut a little deeper into its bed, and a hillside may gradually change as plants establish themselves. Over much longer periods, uplift and erosion alter entire ranges. Understanding mountains requires keeping these different timescales in view and recognizing that even a seemingly solid summit belongs to an active and connected environment.

COVID-19 is a contagious disease caused by the coronavirus SARS-CoV-2. In January 2020, the disease spread worldwide, resulting in the COVID-19 pandemic.

The symptoms of COVID‑19 can vary but often include fever,[7] fatigue, cough, breathing difficulties, loss of smell, and loss of taste.[8][9][10] Symptoms may begin one to fourteen days after exposure to the virus. At least a third of people who are infected do not develop noticeable symptoms.[11][12] Of those who develop symptoms noticeable enough to be classified as patients, most (81%) develop mild to moderate symptoms (up to mild pneumonia), while 14% develop severe symptoms (dyspnea, hypoxia, or more than 50% lung involvement on imaging), and 5% develop critical symptoms (respiratory failure, shock, or multiorgan dysfunction).[13] Older people have a higher risk of developing severe symptoms. Some complications result in death. Some people continue to experience a range of effects (long COVID) for months or years after infection, and damage to organs has been observed.[14] Multi-year studies on the long-term effects are ongoing.[15]

COVID‑19 transmission occurs when infectious particles are breathed in or come into contact with the eyes, nose, or mouth. The risk is highest when people are in close proximity, but small airborne particles containing the virus can remain suspended in the air and travel over longer distances, particularly indoors. Transmission can also occur when people touch their eyes, nose, or mouth after touching surfaces or objects that have been contaminated by the virus. People remain contagious for up to 20 days and can spread the virus even if they do not develop symptoms.[16]

Testing methods for COVID-19 to detect the virus’s nucleic acid include real-time reverse transcription polymerase chain reaction (RT‑PCR),[17][18] transcription-mediated amplification,[17][18][19] and reverse transcription loop-mediated isothermal amplification (RT‑LAMP)[17][18] from a nasopharyngeal swab.[20]

Several COVID-19 vaccines have been approved and distributed in various countries, many of which have initiated mass vaccination campaigns. Other preventive measures include physical or social distancing, quarantining, ventilation of indoor spaces, use of face masks or coverings in public, covering coughs and sneezes, hand washing, and keeping unwashed hands away from the face. While drugs have been developed to inhibit the virus, the primary treatment is still symptomatic, managing the disease through supportive care, isolation, and experimental measures.