venom that can cause injury to humans in the amounts that a spider could inject when biting. While venoms are by definition toxic substances, most spiders do not have venom that is directly toxic (in the quantities delivered) to require medical attention and, of those that do, severity is typically mild. Spider venoms work on one of two fundamental principles; they are either neurotoxic (attacking the nervous system) or necrotic (attacking tissues surrounding the bite). In some cases, the venom affects vital organs and systems. The venoms of the widow spiders, Brazilian wandering spider and Australian funnel-web are neurotoxic. Heart muscle damage is an unusual complication of widow venom that may lead to death. Pulmonary edema, which is fluid accumulation in the lungs, is a feared and potentially serious but uncommon complication of funnel-web venom. Recluse and South African sand spider venoms are necrotic. Recluse venom may also cause severe hemolysis (destruction of red blood cells), though this is typically uncommon. Diagnosis Reliable diagnoses of spider bites require three conditions: first, there should be clinical effects of the bite at the time or soon afterwards, although there are no symptoms universally diagnostic of a spider bite, and bites by some spiders, e.g. Loxosceles species, may initially be painless; second, the spider should be collected, either at the time of the bite or immediately afterwards; and third, the spider should be identified by an expert arachnologist. Spider bites are commonly misdiagnosed. A review published in 2016 showed that 78% of 134 published medical case studies of supposed spider bites did not meet the necessary criteria for a spider bite to be verified. In the case of the two genera with the highest reported number of bites, Loxosceles and Latrodectus, spider bites were not verified in over 90% of the reports. Even when verification had occurred, details of the treatment and its effects were often lacking. Unverified bite reports likely represent many other conditions, both infectious and non-infectious, which can be confused with spider bites. Many of these conditions are far more common and more likely to be the source of necrotic wounds. An affected person may think that a wound is a spider bite when it is actually an infection with methicillin-resistant Staphylococcus aureus (MRSA). False reports of spider bites in some cases have led to misdiagnosis and mistreatment, with potentially life-threatening consequences. Prevention Efforts to prevent bites include clearing clutter