Inferno (Brown novel)
https://en.wikipedia.org/wiki/Inferno_(Brown_novel)
professor Robert Langdon wakes up in a hospital in Florence, Italy with a head wound and no memory of the last few days. Dr. Sienna Brooks, one of the doctors tending to him, reveals that he is suffering from amnesia. When Vayentha, a female assassin, shows up in the hospital and kills Dr. Marconi, Brooks helps Langdon escape, and they flee to her apartment.
After Brooks recounts the details of his admission to the hospital, Langdon finds a cylinder with a biohazard sign in his jacket and decides to call the U.S. consulate. He is told that they are searching for him and want his location. Per Brooks' guidance, he gives them a location across the street from her apartment to avoid getting Brooks more involved in his mysterious situation than she already is. Soon, Langdon sees Vayentha pull up to the location he gave the consulate. At this point, they both believe the U.S. government wants to kill him.
It is discovered Brooks didn't puncture the bag; it was water-soluble and had dissolved one week earlier in the cistern waters, meaning that the whole world has already been infected. The date specified in Zobrist's video was the mathematical calculation of when the entire world would be infected. It is also discovered that she was trying to stop the virus herself, but didn't trust the WHO because samples of the virus would certainly find their way into the hands of governments performing weapons research. The leader of The Consortium tries to escape WHO custody with help from disguised underlings but is caught later by Turkish police. She receives amnesty in exchange for working with the WHO to address the crisis, since she is a medical doctor and has extensive knowledge of Zobrist's research and work.
The plague that Zobrist created is revealed to be a vector virus that randomly activates to employ DNA modification to cause sterility in one third of humans, thereby reducing population growth to a more stable level. Sinskey and Brooks decide not to try to reverse Zobrist's actions, Brooks pointing out that doing so would be difficult and hazardous even for someone like Zobrist and Sinskey acknowledging that Zobrist had a point about the dangers of overpopulation.