"The Giving Plague" is a short story by science fiction author David Brin, published in 1987. It was first published in Interzone and can now be read for free on Brin's personal website.
"The Giving Plague" follows the last few days of a microbiologist only named as Forry, a selfish personality and self-proclaimed cynic, as he struggles to save the remnants of humanity from a plague named TARP, brought home by astronauts during humanity's third manned voyage to Mars.
Written in the form of a diary entry, or maybe an autobiographical note, it details how the narrator's close associate Leslie Adgeson discovers a hereto unknown virus, dubbed ALAS (standing for Acquired Lavish Altruism Syndrome), that is transmitted by blood and manipulates human behavior to enjoy the giving of blood to others (such as via donating blood at the Red Cross) and generally makes them inclined to behave charitably. The narrator, taken aback by the enormity of the discovery, starts to plot the murder of Adgeson in order to take the prestige for himself when disaster strikes in form of an epidemic of the virus CAPUC, over the course of which ALAS is practically forgotten. Adgeson himself dies of the virus, and the narrator takes over his post.
Years later, long after the thus forgotten ALAS had ushered in what appeared as a new age of peace for humanity, the narrator is fighting the outbreak of the aforementioned alien virus TARP, finally close to succumbing to the disease himself. In the final paragraphs, the narrator contemplates how ALAS has destroyed his last trust in human nature, now that he can never be certain if any human decency stems from a true altruistic nature or the manipulations of the virus. He then muses how it was ironically the Acquired Lavish Altruism Syndrome that enabled humanity's current misery by creating the peace that would allow them to finally send manned expeditions to the surface of the red planet.
The short story ends with the narrator cursing ALAS, telling the virus that "...Fever wracks my body now, as I drag myself from bed to bed, holding their hands when they stretch them out to me for comfort, doing what I can to ease their suffering, to save a few. You'll not have me, ALAS. This is what I choose to do."