About 80% of spider bite diagnoses are wrong.

There are many medical conditions that are easily misdiagnosed as spider bites, and there are a number of other arthropods whose bites are blamed on spiders. In one study of 600 suspected spider bites in southern California, 80% were caused by other arthropods, mostly ticks and conenose bugs. Physicians are probably the worst offenders. If the patient implicates a spider and the skin lesion looks like it could be a bite, the doctor usually pronounces it a spider bite, prescribes ointment, and sends the patient home.