"Blood" is the third episode of the second season of the science fiction television series The X-Files. It premiered on the Fox network on September 30, 1994. The teleplay was written by Glen Morgan and James Wong from a story by Darin Morgan, and was directed by David Nutter. The episode is a "Monster-of-the-Week" story, unconnected to the series' wider mythology. "Blood" earned a Nielsen household rating of 9.1, being watched by 8.7 million households in its initial broadcast.

The show centers on FBI special agents Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson) who work on cases linked to the paranormal, called X-Files. In the episode, Mulder and Scully investigate a series of killings in Franklin, Pennsylvania. All the suspects appear compelled to murder after seeing violent messages on electronic devices.

"Blood" was inspired by writer Glen Morgan's own hematophobia as well as controversy over malathion spraying in Southern California. The episode marked the second appearance of the Lone Gunmen in the series, as well as a guest appearance by pornographic actress Ashlyn Gere.

In Franklin, Pennsylvania, postal worker Edward Funsch (William Sanderson) is informed that he will be laid off at the end of the week. Afterwards, Funsch sees the words "Kill 'Em All" on his machine's digital display. At Franklin's civic center, a middle-aged man in a crowded elevator sees "No Air" displayed on the elevator's LED display, and is the only one who can see the message. Sweating and obviously claustrophobic, he again glances at the LED display. This time it flashes the words "Can't Breathe" and then "Kill 'Em All."

Agent Fox Mulder arrives at the civic center in the aftermath of what looks like a massacre. Sheriff Spencer (John Cygan) explains that the suspect murdered four people from the elevator with his bare hands; his rampage ended when he was shot by a security guard. Spencer notes that seven other individuals have murdered twenty-two people in Franklin in the past six months. Mulder discovers that the LED display in the elevator has been damaged, and that the dead suspect has a green residue on his fingertips. Meanwhile, Funsch tries to make a withdrawal from an ATM, but is greeted with the words "Security Guard", "Take His Gun" and "Kill 'Em All" on the screen. He frustratedly beats the screen before running away, escaping from a confused security guard.

At the FBI Academy, Dana Scully reads Mulder's initial report. The only connection between the murders that he can see is that the suspects all destroyed an electronic device during the killings. Meanwhile, Bonnie McRoberts, another Franklin resident, drops by a repair shop to pick up her car, where a message on an engine diagnostic display warns her that the mechanic is going to rape her. She impulsively kills him with an oil can spout. When Mulder and Spencer question McRoberts the next morning, her kitchen microwave instructs her to kill them. When she grabs a knife and attacks Mulder, she is shot and killed by Spencer.

Scully performs an autopsy on McRoberts' body and discovers signs of phobia including high levels of adrenaline and the same substance found on the elevator killer. She hypothesizes that the substance, when combined with other neurochemicals, produces an LSD-like reaction. As Mulder and Scully build a case, Funsch becomes more psychotic, continuing to see violent messages on electronic displays. Blood is associated in some way with each incident; a volunteer asks Ed to donate blood at a department store and seconds later he sees several violent images flash across a sales display of TV sets, followed by a message to get a gun from the sporting goods department.

Late at night, while investigating an orchard, Mulder is sprayed by a crop-dusting helicopter and ends up in the hospital. There, he sees the message "Do It Now" on television and realizes that when people are exposed to the pesticide, which contains a chemical designed to provoke fear in insects, these subliminal messages are relayed to them on purpose and their phobias are exacerbated enough to cause them to kill. Mulder believes the town is being used as a testing ground by a third party, implying that the government is complicit. Eventually, after being confronted, a city councilman agrees to stop the spraying and test the community under the guise of a cholesterol study.