(Portland, OR)  --  Portland is in the top 20 when it comes to the most rat-infested cities.  Orkin ranked the city 20th out of 50 major cities and took into account both residential and commercial rodent treatments.  Chicago ranked number one, followed by New York and Washington, D.C.  The pest control company says fall is the beginning of rodent season, as rats will try to find a warm place to hide from the cold.