Observer Effect is the title of the 87th episode from the television series Star Trek: Enterprise. "Observer Effect" first aired on January 21, 2005 on the American television network UPN.
A viral infection hits Trip while on an away mission with Hoshi in the shuttlepod and he collapses while coughing heavily. Soon, Hoshi begins to exhibit the same symptoms. Upon examination by the ship's medical officer, Dr. Phlox, it is found that the virus is highly contagious and is of a silicon-based life form, rather than a carbon-based one. Archer explains to Trip that T'Pol and Phlox are looking for a cure, taking care not to mention that they were estimated to die within mere hours, given the rate of viral growth. Trip realises the futility of the situation however, since it is basic knowledge that the human immune system is not adapted to fight a silicon-based virus.
The aliens, still residing within the bodies of Malcolm and Travis continue their observation of the human response to this unknown virus. From their conversation, it is understood that the non-corporeal aliens are members of an advanced species who are looking to make "first contact" with other species that have reached a certain level of advancement. A species' response to the unknown virus serves as a standardised test of intelligence. So far, within the 10,000 years of their observation, no species has yet been deemed worthy of first contact. As observers, they are neither responsible for the introduction of the virus, nor are they prepared to save anyone unfortunate enough to become exposed to it. The outcome is always one of two possibilities: Either the virus spreads until the entire crew dies, or those who are exposed to the virus will be isolated or killed to control the spread of the disease, as in the case of a recent Klingon encounter. Initially, the aliens chose Malcolm and Travis as host bodies as they were always present on the bridge and would be kept well informed, and their position as the armoury officer and the helmsman respectively would not interfere with the unfolding events of a medical situation. However, when they failed in their attempts to extract information through casual conversations with Trip, Phlox and Archer, they decided to gain the information first hand by temporarily shifting to the bodies of Phlox and T'Pol.
To keep each other's spirits up, Trip and Hoshi take the time to learn more about each other. Hoshi recounts how during her Starfleet training, she broke a commander's arm after he caught her running poker games on weekends, and was thrown out. However, in light of her exceptional linguistic skills, which was rare and in high demand, Starfleet looked the other way and she was reinstated into Starfleet. Later Hoshi becomes delirious and uses her code breaking skills to break out of the decontamination chamber, thus threatening the rest of the ship with a release of the virus.