What's happened is that by living underground, the subspecies ('forma'), C. pipiens f. MOLESTUS had to adapt to a new primary host. C. pipiens prefers bird blood, but there aren't enough in the subway tunnels to feed on, so f. molestus has adapted to feed on /mammals/ in these subterranean spaces.
London Tube Train ArrivalAlt: London Tube Train Arrival: people stand on the platform motionless as the animated part of the GIF, the tube or subway, moves at a high rate of speed with motion blur shown in the repeating red and white sections.static.klipy.com