Not necessarily. Having a preexisting injury isn't going to destroy your case. When someone causes your injuries, due to their own negligence, they are responsible for the damage they cause.
So long as the most recent accident caused your injuries, and you can show this, you are entitled to full and fair compensation.
Now, your prior injuries and preexisting conditions are important to your case. You need to disclose all of your injuries to your attorney, whether they happened right before the most recent accident or 20 years ago.
Preexisting conditions need to be disclosed so that an analysis can be done. We need to determine if your present symptoms are caused by the most recent accident. Maybe it is a mix of existing symptoms that are made worse by the most recent accident.
Also, preexisting conditions can make your case better. Prior injuries make you more susceptible to injury to those sections of your body. If you already have back problems, and get hit by another car or fall down, your back might be in worse shape than someone else who didn't have any existing injuries there.
It's only fair to analyze your case in this manner. If the negligent person who caused your injuries was only responsible for 50% or 70% of your present injuries, then that is how much they should be responsible for. Also, if your prior injuries made it easier for you to suffer greater injuries, then the defendant should be responsible for that.
So, it is important that you are not afraid to tell your doctors and attorney about any prior injuries, accidents or conditions you have. If your pain or symptoms are worse after an accident than before, than it'll be shown that the defendant caused you injury, and owes you full and fair compensation.
California has two relevant jury instructions on pre-existing conditions.
[Name of plaintiff] is not entitled to damages for any physical or emotional condition that [he/she] had before [name of defendant]'s conduct occurred. However, if [name of plaintiff] had a physical or emotional condition that was made worse by [name of defendant]'s wrongful conduct, you must award damages that will reasonably and fairly compensate [him/her] for the effect on that condition.
Sources and Authority
"A tortfeasor may be held responsible where the effect of his negligence is to aggravate a preexisting condition or disease." (Hastie v. Handeland (1969) 274 Cal.App.2d 599, 604 [79 Cal.Rptr. 268], internal citations omitted.)
"Plaintiff may recover to the full extent that his condition has worsened as a result of defendant's tortious act." (Ng v. Hudson (1977) 75 Cal.App.3d 250, 255 [142 Cal.Rptr. 69], internal citations omitted, overruled on another ground in Soule v. General Motors (1994) 8 Cal.4th 548, 574 [34 Cal.Rptr.2d 607, 882 P.2d 298].)
"It is by no means self-evident that an act which precipitates a flare-up of a pre-existing condition should be considered a 'cause which, in natural and continuous sequence, produces the injury.' Thus, general instructions on proximate cause were not sufficient to inform the jury on the more specific issue of aggravation of pre-existing conditions." (Ng, supra, 75 Cal.App.3d at p. 256.)
"[An instruction on preexisting condition] is proper only where the injured is the claimant seeking compensation for his injuries. That is not the case here in a wrongful death action." (Vecchione v. Carlin (1980) 111 Cal.App.3d 351, 358 [168 Cal.Rptr. 571].)
You must decide the full amount of money that will reasonably and fairly compensate [name of plaintiff] for all damages caused by the wrongful conduct of [name of defendant], even if [name of plaintiff] was more susceptible to injury than a normally healthy person would have been, and even if a normally healthy person would not have suffered similar injury.
Sources and Authority
"That a plaintiff without such a [preexisting] condition would probably have suffered less injury or no injury does not exonerate a defendant from liability." (Ng v. Hudson (1977) 75 Cal.App.3d 250, 255 [142 Cal.Rptr. 69], internal citations omitted, overruled on another ground in Soule v. General Motors (1994) 8 Cal.4th 548, 574 [34 Cal.Rptr.2d 607, 882 P.2d 298].)
"The tortfeasor takes the person he injures as he finds him. If, by reason of some preexisting condition, his victim is more susceptible to injury, the tortfeasor is not thereby exonerated from liability." (Rideau v. Los Angeles Transit Lines (1954) 124 Cal.App.2d 466, 471 [268 P.2d 772], internal citations omitted.)
As you see, even if you have pre-existing injuries, or you were more susceptible to injury, you can still recover fair compensation from the person who caused your injuries.
But your attorney needs to know about these issues before the defendants and their attorneys do. We can usually deal with pre-existing issues as long as we know about it.
So you must absolutely tell your personal injury attorney about any possible issues.