The problem with cancer cells is that they are not foreign invaders, but fifth-column traitors.

FORBES: In search of antigens

Even though it requires the illegal alien's 100 days of agricultural work in the U.S. to have occurred during any 12 month period between February 2002 and August 2003 - and, therefore, is not something new "invaders" could cash in on - this legislation further reinforces the expectation that, if you can get into this country by whatever means, you will at some point likely be allowed to stay legally.

CENTERFORSECURITYPOLICY: Amnesty by any other name

The fashion then was for collection and growing foreign species so the Spanish invaders going native was not seen as a problem.

BBC: NEWS | UK | Northern Ireland | Gardeners urged to repel boarders