The paper describes cases of pseudoinvasion and transient carriageof the larvae of arthropods and geohelminths. It highlights the objective and subjective factors in the diagnosis of arthropod-borne diseases and geohelminthiasis, which give rise to misdiagnosis. Distinctions between real invasion, transient invasion, and pseudoinvasion are pointed out. The fact that there are special problems with the misdiagnosis of people with mental diseases is emphasized. The key factors for preventing and diagnosing pseudoinvasions and transient carriage are defined.