Severus Snape killed Albus Dumbledore at the top of the Astronomy Tower in Harry Potter and the Half-Blood Prince.
Snape cast the killing curse at Dumbledore's own request, which served several specific purposes:

• A Mercy Killing: Dumbledore was already dying a slow, agonizing death from a cursed ring and wanted to be spared a miserable end.
• Protecting Draco Malfoy: Voldemort ordered Draco to kill Dumbledore. Dumbledore asked Snape to do it to save Draco from the soul-destroying burden of murder.
• Gaining Voldemort's Trust: By killing Dumbledore, Snape completely convinced Voldemort of his loyalty, allowing him to secretly continue protecting Harry and the students from within the Death Eater ranks

An infectious particle that can only reproduce inside a living cell is called a virus.  Because they cannot replicate on their own, viruses operate as obligate intracellular parasites. They survive by hijacking a host cell's metabolic and reproductive machinery to assemble new viral particles. The complete, fully assembled, and infectious form of the virus outside of a host cell is known as a virion.

The lyric is from the Eagles' 1972 hit debut single, "Take It Easy". Co-written by Jackson Browne and Glenn Frey, the song put the small town of Winslow, Arizona, on the map, inspiring the actual Standin' on the Corner Park built there to commemorate the lyrics. 

New York was the first US state to require license plates, passing a law in 1901 that mandated motor vehicle registration. Because the state did not provide official tags, motorists were originally responsible for making their own plates by displaying their initials on the back of their vehicles. 

South Carolina was the first state to secede from the Union, doing so on December 20, 1860. A state convention unanimously passed an Ordinance of Secession following the election of Abraham Lincoln. This initiated the wave of disunion that ultimately led to the American Civil War.