Short answers, pulled from the story.
The brown rat is thought to have originated in the plains of northern China and Mongolia. Despite its scientific name Rattus norvegicus and common names like Norway rat, it did not originate from Norway; the English naturalist John Berkenhout popularized that misnomer in his 1769 book Outlines of the Natural History of Great Britain.
A female brown rat can produce up to five litters per year, with each litter numbering up to 14 pups after a gestation of just 21 days. Pups reach sexual maturity in about five weeks, meaning a population can theoretically grow from two individuals to around 15,000 in a single year.
Brown rats can carry pathogens associated with Weil's disease, rat bite fever, cryptosporidiosis, viral hemorrhagic fever, Q fever, hantavirus pulmonary syndrome, and toxoplasmosis. During the COVID-19 pandemic, a study of New York City sewer rats found that 17 percent of the sampled population had been infected with SARS-CoV-2.
Alberta launched an aggressive rat-control program in 1951 backed by legislation requiring all residents and municipalities to destroy rat populations. The province's geography helps: boreal forest, mountains, and semiarid plains block natural entry routes except for a narrow eastern border with Saskatchewan. By 2002, Alberta recorded its first year with zero infestations.
Brown rats produce ultrasonic vocalizations beyond human hearing, including a chirping sound during play and tickling that researchers have likened to laughter and associated with positive emotional states. They also produce audible sounds like bruxing, or teeth-grinding, most commonly triggered by contentment.
Wild urban rat populations began evolving resistance to the anticoagulant rodenticide warfarin by the mid-20th century. Some populations in the United Kingdom have also developed resistance to the second-generation rodenticides created to replace warfarin. A behavioral shift toward fear of novel stimuli has made effective bait placement harder as well.