The Andromeda Strain

Main Control Lab at the Wildfire complex
Two crewmembers of the U.S. government's "Project Scoop" investigate the town of Piedmont, New Mexico, to retrieve the Scoop 7 satellite. They find townspeople dead in the streets while reporting back to Scoop Mission Control. Suspecting the satellite may have brought back an extraterrestrial pathogen, Scoop Mission Control activates an elite scientific team it had previously recruited for just this type of emergency.
Nobel laureate Dr. Jeremy Stone (Arthur Hill), the team leader, and Dr. Mark Hall (James Olson), the team surgeon, are dropped in Piedmont by helicopter, where they search the town for Scoop 7 in hazmat suits. They find the town's doctor, who died after opening the satellite out of curiosity. Hall cuts open the doctor's corpse and finds that all of his blood has clotted and turned to powder. Stone and Hall retrieve Scoop 7 and find two survivors â a 62-year-old man and an infant (who "can't be more than 6 months old").
The entire team of four core research scientists, including Dr. Charles Dutton (David Wayne) and Dr. Ruth Leavitt (Kate Reid), are summoned from their academic and research appointments to arrive at a top secret, high-tech underground laboratory in Nevada, named Wildfire, where they undergo a full day of decontamination procedures, descending through five rigorous disinfection levels of the lab. Before decontamination, Hall is informed that the facility's nuclear device has been armed and will go off automatically within five minutes should there be a containment breach. He is entrusted with a key to disarm the bomb from designated substations within that window. The team enter the facility under the impression that Piedmont has already been sterilized by a nuclear bomb.
The team begins to experiment on the agent inside Scoop 7 and discovers that it is transmitted by air, kills lab animals instantly, and is too large to be a virus. After searching the satellite with a high-powered microscope, they discover the agent responsible for the deaths: a greenish, throbbing mass stowed away on a micrometeorite. It receives the code name "Andromeda".
While most of the team studies the agent in an attempt to figure out how it works, Hall tries to find a cure by figuring out why the old man and the baby survived. Both Hall and his nurse assistant work in encapsulating suits while in the contaminated "hot room". By analyzing their patients' blood and conferring with Peter Jackson (the old man, played by George Mitchell), they discover that he has abnormally acidic blood from drinking "Squeeze" (Sterno) to relieve his stomach ulcer. No anomalies are found in the baby.
Meanwhile, a fighter jet pilot, flying over Piedmont at high altitude, notices his "rubber" mouthpiece disintegrating. He then loses consciousness and crashes. The military inspects the wreck and finds that the pilot's flesh has dissolved, leaving only bones. While discussing the pilot's claims that all the "rubber" was dissolving, they are told that there is no actual rubber aboard a Phantom F-4, instead the material is "Polycron," a synthetic substance with properties similar to human skin. Unknown to the others, Leavitt's research is impaired by attacks of absence epilepsy, which is set off by the flashing red "No Growth" indicator on the Petri dish analyzer. This causes her to report, erroneously, that Andromeda grows on every growth medium.
During chemical study of the agent, the team finds out that the meteorite is in fact made of plastic and that the green mass is a lifeform of similar chemical composition to Earth life. However, it lacks amino acids, enzymes, or proteins. They then discover that Piedmont has in fact not yet been bombed due to the caution of the President, and that a fighter flying over Piedmont crashed. They again insist the bomb be dropped. Further study of Andromeda, however, reveals that it has a crystalline structure which allows it to convert energy to mass and vice versa, consuming any available resource without waste. They deduce that a nuclear explosion would provide Andromeda with enough energy to produce a supercolony in a single day. Reversing their opinion, they now insist the Piedmont bombing be stopped.
During analysis, the team comes across a germ warfare simulation, indicating that Scoop and Wildfire were designed to actively search for harmful biological agents for use in bio-warfare. Meanwhile, Andromeda mutates into a form that degrades synthetic rubber and plastics and thus escapes from the containment room adjacent to where Dutton is working, trapping him in an atmosphere laced with the agent. Hall rescues Leavitt from a photosensitive epilepsy seizure triggered by Wildfire's alarm system.
Hall continues his work on the survivors and finds out that Andromeda propagates only within a narrow pH range and that the baby's rapid breathing kept its blood alkaline and therefore inhospitable for the organism. Jackson's blood, suffering from acidosis, was just the opposite: its low pH also made it inhospitable for the organism. He advises Dutton to breathe rapidly as well, but then observes a rat in good health alongside Dutton, meaning that the organism has mutated and become benign to terrestrial life.
Almost immediately, after Stone notes that problems with gasket deterioration at the lunar lab induced them to use Polycron for the gaskets in Wildfire, Andromeda starts decaying all of the lab's seals, activating the nuclear self-destruct countdown, and trapping the team on the fifth (lowest) level. Hall races against the clock to reach a substation with his key. With the doors sealed, he climbs ladders in a maintenance access core and endures an attack by automated lasers intended to stop escaped lab animals, until at last he finds a working third level substation, disables the bomb, and passes out.
The military then begins seeding the clouds over the desert with silver iodide, stimulating precipitation which washes the organism into the ocean, where they believe it will be destroyed by the alkalinity of salt water. A federal board concludes that the crisis is over, but Stone somberly asks what may happen if such a situation were to recur.
The film ends with a close up of the virus as it mutates again, causing the computer to overload, showing that Andromeda remains unpredictable.

Question: What's seeded in the clouds to cause precipitation?
Answer: Silver iodide.; Silver iodide.