Personal injury law is designed to make people whole again. Juries are supposed to take a look at how the plaintiff was doing before the wreck, and then evaluate how they have changed. Most people have pre-existing conditions, health problems and prior injuries. This is not a big deal. Everyone had some sort of injury in their lifetime. Juries are just supposed to remove any damages that existed before a new injury.
For example, a plaintiff had prior knee problems but then had a surgery that fixed it. The plaintiff still had some occasional weakness or pain, but most of the time they were fine. If the plaintiff gets injured, then the defendant is responsible for all new injury. You can certainly have a new injury to a body part that was injured in the past. The jury is just supposed to remove any problems that were there before, which is called pre-existing medical conditions.
While this is not that complicated, defense attorneys and insurance companies will make a very big deal out of it. They will argue that all of the problems are pre-existing medical conditions. They will suggest that the plaintiff was actually injured before, even if there are no doctor visits or any evidence of any problems. Without coming out and saying it, they will accuse you of lying. You will need to basically call your doctor, some family, coworkers and other people who know you to testify. They will simply describe your functioning before and after a car wreck.
Even with this evidence, your attorney will need to be careful selecting a jury. While jurors are supposed to only base their verdict on the evidence, some ignore this law. Some jurors will bring personal bias against lawsuits or plaintiffs and allow this to influence their decisions. You will need to work hard to make sure that you get jurors that we not allow these out of court concerns influence the verdict. The law doesn't allow it, and you must make sure that they will follow the law. Sometimes, it is hard for them to set aside personal feelings, even when the judge instructs them that they must. Ultimately, pre-existing conditions is a normal part of life and a normal part of a trial. With hard work and proper presentation, the jury will simply reduce the verdict by the amount of prior problems.