Portugal has arguably the most liberal laws concerning possession of illicit drugs in the Western world. In 2001, Portugal decriminalized possession of effectively all drugs that are still illegal in other developed nations including, but not limited to, cannabis, cocaine, heroin, and LSD. While possession is legal, trafficking and possession of more than "10 days worth of personal use" are still punishable by jail time and fines. People caught with small amounts of any drug are given the choice to go to a rehab facility, and may refuse treatment without consequences. Despite criticism from other European nations, who stated Portugal's drug consumption would tremendously increase, overall drug use has declined along with the number of HIV infection cases, which had dropped 50 percent by 2009. Drug use among 16- to 18-year-olds also declined, however the use of marijuana rose only slightly among that age group.

Answer this question, if possible (if impossible, reply "unanswerable"): By 2009, by how much had the rate of HIV infection decreased?
50 percent