ENT Season 4 Episode 11 Observer Effect

Plot

Lieutenant Reed (Dominic Keating) and Ensign Mayweather (Anthony Montgomery) play chess while serving as hosts to non-corporeal alien known as Organians. Returning from an away mission on the planet below, Commander Tucker (Connor Trinneer) and Ensign Sato (Linda Park) soon exhibit symptoms of a strange disease. Upon examination by Doctor Phlox (John Billingsley), it is found to be a highly contagious silicon-based virus – which carbon-based physiology cannot counteract. To pass the time, while isolated in Decontamination, Tucker and Sato try to learn more about each other.
The Organians are keenly interested in examining the human response to this crisis, and compare notes to previous reactions by Klingons and Cardassians. They are members of an advanced species looking to make "first contact". For 800 years, the pair have been passively observing various space-faring species as they react to the pathogen, but no species has yet been deemed ready. Seeking a different view of the crew, they variably shift to the bodies of Captain Archer (Scott Bakula), Commander T'Pol (Jolene Blalock) and Phlox. The two Organians start to disagree: one seems determined to maintain their non-interference protocol, while the other feels the protocol is outdated and unnecessary.
With time running out, Phlox and T'Pol find a way to disrupt the virus using deadly levels of radiation. Archer and Phlox, while wearing environmental suits, escort Tucker and Sato to Sickbay for treatment. Sato soon goes into cardiac arrest, and Archer removes his gloves and helmet to assist her, but she cannot be resuscitated. They then administer a dose of radiation to Tucker, but he dies too.
Suddenly, the Organians possess Tucker and Sato, explaining the situation to a surprised Archer, including the unstoppable spread of the infection. Archer pleads on behalf of his crew, pointing out that the Organians have lost empathy, confusing non-intervention after-the-fact with a harmful choice to not post warnings about the virus. They decide to modify their protocol, choosing to resurrect and cure the infected crew members, when they previously would have left the entire crew to die. They erase the encounter from the crew's memory. Archer orders a warning beacon to be placed above the planet, and the Organians leave to begin planning first contact with humans 5,000 years in the future.