The term "emerging diseases" is a loosely defined category of entities comprising resurgent or recurrent old diseases (usually caused by "new" or mutated previously known agents), diseases truly new to man, but caused by preexisting ("old") zoonotic agents, and syndromes newly defined by the discovery of new agents through advances in biotechnology. Identification and solution of these problems depends, first, on recognition of their differences, and then upon tailoring appropriate strategies for their control. Thus, new influenza viruses appear each year to challenge immunity to their antecedents, but evoke the unchanged and centuries old symptom complex of influenza. Tuberculosis, is resurgent because of mycobacterial mutation to antibiotic resistance, immunosuppression by AIDS, and laxity in public health surveillance. Parvovirus B19 and herpesvirus 6 were revealed as cryptic infectors of white blood cells in studies of hepatitis B and AIDS, but since have been shown to be important causes of childhood rashes, aplastic anemia, and neurologic disease. The encroachment of human habitation on wilderness perimeters (ecosystem change) has increased contact with vectors of zoonotic viruses and bacteria, as evidenced by Lyme disease, Ebola virus infection, and the hemorrhagic fevers. The term "holistic epidemiology" embraces all these problems, from the molecular to the macroenvironmental level. Humans, parasites, and their environment will continue their ancient, fluctuating, dynamic relationship in the future, and new diseases will continue to emerge.