Prey by Michael Crichton

The plot of the book is that in a Nevada desert laboratory, an experiment has gone wrong and a cloud of micro robots - nanoparticles has escaped in to the air. These particles are self replicating and self sustaining. 

One software engineer  Jack Forman gets fired discovering an internal scandal. He becomes a house husband of Julia who is an employee of Nano biotics industry. She spends most of her time on the research which Jack misinterprets as her affair with someone. One day, she meets with an accident and jack is asked to take her role at office since her employer is having software issues with the product. 

Jack is given a tour of the lab and meets the programming team. He is shown a very complicated machine used to make the nanobots. Ricky refuses to show Jack the source code for the nanobots, and later Ricky claims that building contractors failed to properly install filters in a certain vent in the building. As a result, hazardous elements such as the assemblers, the bacteria, and the nanobots were blown into the desert, evolving and eventually forming autonomous swarms. These swarms appear to be solar-powered and self-sufficient,reproducing and evolving rapidly. The swarms exhibit predatory behavior, attacking and killing animals in the wild, using code that Jack himself worked on. Most alarmingly, the swarms seem to possess rudimentary intelligence, the ability to quickly learn and to innovate. The swarms tend to wander around the fab plant during the day but quickly leave when strong winds blow or night falls.

The nanoswarm kills a rabbit outside the complex, and Jack goes outside with Mae to inspect. They find that the rabbit died of suffocation resulting from the nanobots blocking its bronchial tubes. While Mae goes inside for equipment, Jack is attacked by the swarms. He barely manages to get through the airlock inside the lab before falling unconscious from anaphylactic shock.

Persuaded by Jack, the team decides to destroy the swarm. They believe that the swarm must have nested in the desert to reproduce. They attempt to find this nest by tagging the swarm with radioactiveisotopes and following them back to their nest at night. Under the cover of a strong wind that forces the swarms to remain dormant, the team goes outside to a storage shack to find the isotopes and build a spray device. However, as the wind dies down, four swarms attack the shack and eventually kill David and Rosie. The rest of the team are forced to take shelter in the cars parked outside. The Swarms begin an attempt to enter the cars.

Eventually, the swarms find a way to enter the cars, but not long before the wind picks up in speed again. Jack and Mae manage to escape to the lab before losing consciousness, but Charley falls unconscious outside his car after he sprays his swarm with the isotope. Bobby, Vince and Ricky refuse to go outside and help Charley. Jack, dizzy and nauseous, goes back out again to save Charley as the swarms attacks again. Using a motorbike found in David's car, Jack manages to get himself and the semi-conscious Charley to the safety of the airlock before he falls unconscious again.

As night falls, Jack, Mae and Bobby set out to find the swarms. While searching for them, they discover that one of the swarms, now so evolved that it can operate without solar energy, is moving the now deceased Rosie through the desert. They follow the body to find the swarms nesting in a cave. As some of the swarms come out of the cave after them, a Xymos helicopter arrives and traps the swarms inside the cave using its powerful draft. Mae and Jack then venture into the cave and proceed to exterminate the swarm, their nest and their organic assembly plant (which looks very similar to the original Xymos assembly plant) using explosive thermite caps. They return to the Xymos plant, exhausted.

At the plant, Jack, Mae and Bobby are enthusiastically greeted by Julia, who was earlier discharged from the hospital and was brought in by the chopper. Julia's behavior seems to be extremely aberrant: She seems to pay heed to nothing other than trying to entice Jack and kissing him, even when Charley is found dead in the locked communications room with a swarm flying around him and the communication links cut. Jack cannot understand how the swarm got inside the rigorously protected airtight building, why Charley would have disabled the facility's communications, or why Julia and Ricky seem to be coming up with various out-of-character ways of how he died.

To Jack's horror, the video not only reveals that Julia and Ricky had an affair but also shows how Charley engaged in a vicious fight with Ricky and Vince. All of them end up in the communications room where Julia kisses a subdued Charley, injecting a stream of swarm into his mouth.

Eventually, Jack and Mae realize that everyone in the facility except themselves have been infected by a symbiotic version of the nanobot swarms. These nanobots, although evolved alongside the other swarms, do not show aggressive predatory behavior. Instead, while they seem to invigorate their hosts' physical statistics and their perception, they slowly devour and take over their hosts, initially affecting their decisions and then controlling them, while allowing them to travel and contaminate others.

Jack comes up with a plan to destroy this new strain. Mae and Jack drink vials containing a form of phage that kills the nanobot-producing E. coli bacteria. The phage would protect them from infection. Jack then proceeds to take a sample of the phage and pour it into the sprinkler system and drench everyone with it. He tricks Mae into alerting Julia and the infected team. They set out to stop Jack. In the vicious struggle that ensues, Vince is killed and Jack, who barely escapes death several times, finally manages to place the sample into the sprinkler system.

In order to prevent the sprinkler system from triggering, infected-Ricky disables the plant's safety network. However, this is exactly what Jack wants, as Mae has already allowed the phage into the assembly line, causing the phage to reproduce rapidly. The assembly line is rapidly overheating because of the no longer active safety system. If Ricky and Julia do not turn on the safety system the assembly line will burst, filling the lab with the phage. The infected-team, who are now doomed either way, choose to re-activate the safety network and get drenched with the phage. Jack and Mae escape the facility in a helicopter shortly before the facility explodes due to a methane gas leak combined with thermite Mae has placed in the building. After returning home, Jack infects all his children with the phage to eradicate the potential nanobot infestation. Mae calls the U.S. Army and sends a sample of the phage to her lab.

Jack puts together all the missing links. The corrosion of the memory chip in Eric's MP3 player as well as Amanda's rash were caused by gamma assemblers. The MRI's strong magnetic field detached the assemblers from her. These assemblers were most likely brought home by Julia. Knowing this, Julia called in the Xymos special team to scan Amanda's room. The person who Jack spotted in Julia's car was in fact the cloud of nanobots. Xymos intentionally released the swarm into the desert so that it would evolve to stay in a cohesive group in the wind.

Gives a thrill and keeps you occupied for hours.