A "statute of limitations" is a law that sets a time limit on your right to file a lawsuit. The idea behind these laws is that prospective plaintiffs shouldn't be allowed to wait an unreasonable amount of time before taking their case to court.
If you try to file a lawsuit after the statutory time limit has passed, the court will refuse to hear the case, unless an exception applies. Different kinds of cases have different deadlines. In most states, medical malpractice cases are subject to their own specific statute of limitations.
California's statute of limitations for medical malpractice lawsuits can be found at California Code of Civil Procedure section 340.5, which states that this kind of case must be brought "within one year after the plaintiff discovers, or through the use of reasonable diligence should have discovered, the injury," or within three years of the date of the injury, whichever comes first.
In other words, in California, once you learn that you were harmed by a health care provider's negligence, you need to get your lawsuit filed in the state's civil court system within a year of that discovery. And if you don't discover that you were harmed until after more than three years have passed since the negligent act occurred, you will have lost your right to file a medical malpractice lawsuit in California.
One exception to this overall three-year deadline is cases where a foreign object—such as a medical instrument or a surgical sponge—was left in the patient's body. In those kinds of cases, the one-year discovery deadline still applies, but there is no overall time limit. So you could bring this kind of case ten years or more after the surgical error occurred, as long as you file it within a year after you find out about the presence of the foreign object.
Another exception is when the plaintiff is younger than six years old. Even if more than three years have passed since the injury, the medical malpractice lawsuit may be filed as long as it's before the child's eighth birthday.
Also, the statute of limitations is "tolled" (meaning that the time period is paused) under certain circumstances, including when the defendant intentionally hid the malpractice or committed fraud.
Finally, it's important to take note of one more procedural hoop that California medical malpractice plaintiffs must jump through: California Code of Civil Procedure section 364 entitles the defendant healthcare provider to at least 90 days' notice of a patient's intent to file a medical malpractice lawsuit. If the plaintiff provides that notice within 90 days before statute of limitations runs out, the deadline will be extended for 90 days from when the notice was served on the defendant.