Generally speaking, a personal injury claim can be filed in California when a person is injured due to the fault of someone else. Of course people are injured every day and it is no one else's fault. A person might trip over his own shoes, or fall because he is carrying more than he can reasonably carry and maintain his balance.
Also, in order to file a personal injury claim in California, an injured person must be able to prove that someone owed him a duty, breached that duty, and the breach was the cause of the injury. While the legal language seems complex, it is actually not as complicated as it might sound.
For example, a business owner owes a duty to every person who comes into his business to provide a safe environment, including customers and employees. If that business owner doesn't keep the sidewalks or aisles in his store clear of debris and hazards and someone falls on it and is injured, then he may be liable for the resulting injury.
Another example is a doctor owes his patient a duty to provide competent medical care. When a doctor does not provide competent medical care and a patient is injured as a result of his incompetence, or perhaps carelessness, then the patient may file a personal injury lawsuit for the doctors negligence.
Someone who is injured due to a driver's failure to follow the rules of the road may file a personal injury lawsuit against that negligent driver. However, if a person is injured due to his own careless driving, then he cannot file a personal injury lawsuit.
Obviously a person who has just a little pain or has a bruise, but is otherwise not injured, would not have medical bills and would probably not have much of a claim for damages. A personal injury claim is usually filed where there are damages such as medical bills or lost wages which need to be compensated. A person who has pain that prevents him from working though, even for a couple days, can file a personal injury claim to recover the money he lost from missing work.
If an employee is injured while working, he would usually file a worker's compensation claim rather than a personal injury claim. A person who is injured outside of work though, and then misses days from work can file a personal injury lawsuit to be compensated for missed time. There are limits to recovery in workers compensation cases and sometimes injured workers cannot choose their own doctors. It can be more beneficial to an injured party to file a personal injury claim than a workers compensation claim. An experienced personal injury attorney can assess your case and determine whether a personal injury claim would be appropriate in your particular case.