"Observer Effect" is the eleventh episode of the fourth season of the American science fiction television series Star Trek: Enterprise and the eighty-eighth overall. It was first aired on January 21, 2005, on UPN. It was written by Judith and Garfield Reeves-Stevens, and directed by Mike Vejar. Set in the 22nd century, the series follows the adventures of the first Starfleet starship, Enterprise, registration NX-01. This episode sees alien entities test the Enterprise crew by observing their reactions to a deadly silicon-based infection. Actual first contact with these aliens – the Organians – would occur about a century later, during the events of the Star Trek: The Original Series episode "Errand of Mercy".