Ichthyophthiriasis (white spot disease) is an economically important protozoosis caused by Ichthyophthirius multifiliis in freshwater fish. Medication prevention and curation are the main methods to control this disease with vaccines in laboratory, but the efficacy of drugs practically acts on the free-living (nonparasitic) stage of I. multifiliis and can be easily impaired by a variety of environmental factors. Thus, study on the biological properties of I. multifiliis and the complicated ecological relationships between I. multifiliis and other biotic or abiotic factors that influence epidemicity of ichthyophthiriasis will contribute to integrated control of ichthyophthiriasis. In this article, some I. multifiliis biological properties, such as systematic position and life cycle of I. multifiliis, are briefly reviewed, and the seemingly abnormal phenomenon associated with in vitro cultures is specially discussed; then, the epizootiology of ichthyophthiriasis is emphasized, which involves various biotic or abiotic factors that impact the life and action of I. multifiliis. The susceptibility and stress reaction of fish to I. multifiliis infection are stated. Also, the pathogenicity and diagnosis of ichthyophthiriasis were covered, and an overall assessment is finally made on ichthyophthiriasis control.