Short answers, pulled from the story.
Citrus originated in the foothills of the Himalayas, in a region spanning eastern Assam in India, northern Myanmar, and western Yunnan in China, according to a DNA study published in Nature in 2018. The genus diverged from a common ancestor about 15 million years ago. A climate shift during the Late Miocene then triggered rapid speciation into the many wild varieties known today.
Almost all commercially important citrus fruits, including oranges, lemons, grapefruit, and limes, are hybrids derived from three ancestral species: the mandarin orange, the pomelo, and the citron. Hybridization between these species and other wild Citrus relatives occurred within the last few thousand years.
The citron arrived in the Mediterranean around 1200 BCE, based on seeds recovered from the Hala Sultan Tekke site in Cyprus. Lemons, pomelos, and sour oranges were introduced by Arab traders around the 10th century CE. Sweet oranges were brought to Europe by Genoese and Portuguese merchants during the 15th and 16th centuries.
Grapefruit contains furanocoumarins, chemicals that interact with medications when taken orally, a phenomenon known as the grapefruit juice effect. These compounds interfere with how the body processes certain drugs. The same class of compounds also causes phytophotodermatitis, a skin inflammation, when applied topically and followed by ultraviolet light exposure.
The UN Food and Agriculture Organization recorded world production of all citrus fruits at 124 million tonnes in 2016, with approximately half of that being oranges. By 2019-20, orange production alone was estimated at 47.5 million tonnes, led by Brazil, Mexico, the European Union, and China. Citrus trade in 2018 was valued at US$15.2 billion, making up nearly half of world fruit trade that year.
Citrus greening disease, caused by a bacterium spread by the Asian citrus psyllid, threatens production in Florida, California, and worldwide. Other major threats include citrus canker (bacterial), citrus black spot (fungal), sweet orange scab caused by Elsinoe australis, and the citrus tristeza virus transmitted by aphids. Parasitic nematodes, including Tylenchulus semipenetrans, also attack citrus groves.