Dr. Emil Hobbes is conducting unorthodox experiments with parasites for use in transplants. He believes that humanity has become over-rational and lost contact with its flesh and its instincts, so the effects of the alien organism he actually develops is a combination of aphrodisiac and venereal disease. Once implanted, it causes uncontrollable sexual desire in the host.
Hobbes implants the parasites in his teen-aged mistress, who promiscuously spreads them throughout the ultra-modern apartment building outside Montreal where they live. Hobbes, unable to undo the damage he caused, kills his mistress and then commits suicide. The police are called and the crime looks to be open and shut.
As the story develops, one of Hobbes' sexual partners begins to feel ill and returns from work. Here we see the parasite emerge from its host and escape into the building where it emerges and attacks a number of people. The pace of the story quickens when the community's resident physician, Roger St. Luc, uncovers some of the research that Hobbes had been working on. St. Luc encounters an elderly resident who has been attacked and burned by the parasite. St. Luc, along with his assistant and girlfriend, Nurse Forsythe, move the elderly residents to their room. They attempt to stop the parasite infestation before it overwhelms the city's population.
Instructing the elderly couple to wait and lock themselves in, St. Luc continues to the basement where the residents told him they had disposed of the parasite. St. Luc is attacked by a caretaker but manages to defeat him by bashing in his skull. Forsythe leaves the safety of the elderly residents' room and continues after St. Luc, where she is attacked, but is rescued by St. Luc. Meanwhile, upstairs it is clear that the parasite has spread the infection as more and more of the residents start to act out. The scene closes with the elderly couple's apartment broken into by the infected. Downstairs the security guard is infected and the auctioneer showing the apartment is slowly entrapping other unsuspecting guests.
St. Luc escapes to the parking garage where Forsythe is attacked by an infected resident. St. Luc rescues her and takes her to his car. However, as they attempt to crash through the gate to the parking garage another car rams them. St. Luc helps Forsythe free and they escape to a remote area in the building. At this stage Forsythe starts to act out, showing that she too has become infected. St. Luc is forced to leave her and forge on to escape but at every turn he is trapped. Eventually he finds himself trapped in the swimming pool and he is attacked and eventually infected by Forsythe.
The closing scene is the residents happily exiting the residential block in their cars. The viewer is left to believe that Hobbes' plan to infect the world is underway.

Question: Why are the residents acting ou?
Answer: They have been infected by the parasite.
Alternative answer: attacked by parasites