Think of a cracked windshield: a small impact that might barely mark one car can cause major damage in another that was already fragile. The eggshell plaintiff rule works in a similar way. A person who causes harm must take the injured person as they find them, including any preexisting physical, neurological, or psychological vulnerability. If the same event causes worse injuries because the injured person was more susceptible than an average person, the at-fault party can still be responsible for the full extent of that added harm.

In practice, this rule matters when a crash, fall, or blow to the head turns a manageable condition into a serious one. A mild collision may trigger severe symptoms in someone with a prior brain injury, migraine disorder, spinal weakness, or anxiety condition. The defense usually cannot avoid liability just by arguing that a healthier person would have recovered faster or suffered less. The key question is whether the incident actually worsened the person's condition, not whether the person was unusually vulnerable.

For an Indiana injury claim, the rule often affects the value of damages, especially medical treatment, lost function, and future care. It does not erase comparative fault. Under Indiana's Comparative Fault Act (1985), a claimant generally cannot recover if they are more than 50% at fault, and any recovery is reduced by their share of fault.