Suddenly emerging diseases have always occurred. Yersinia (plague) and Treponema (syphilis) caused great alarm, and in the Middle Ages nearly one-third of all living humans were killed by plague epidemics that swept Europe. In the early 1990s influenza became a major health threat; and in the 1980s AIDS and lume disease became epidemic diseases. Current emerging diseases include "new" diseases and the resurgence of "old" diseased related to failed public health efforts and antibiotic resistance.

Several factors have been responsible for the emergence of new pathogens. Currently nearly one-half of the world's population lives in cities. The population density of modern cities makes the transmission of pathogens much easier. Sexual activity and drug use have increased the spread of some diseases. Antibiotic resistance by microbes has increased recently. Changes in land use (destruction of forests, dams, and building houses near forests), bulk transportation of foods, international travel, unusual exposure to wild animals, breakdown in publc health measures, and inadequate vaccination programs are some other reasons for the emergence of new infections/diseases.

Many microbiology textbooks list nearly 40 new emerging diseasess; and viruses are thought to be the most numerous emerging pathogens. Among the emerging and resurging bacterial deseases are TB, lyme disease, toxic shock syndrome, food borne illnesses, cholera, legionnaire's disease, and necrotizing fasciitis. Suppression of immune system and over-crowding appear to be the reasons for the resurgence of Mycobacterium (TB). The increase of deer populations and human populations moving to the forests have provided means for the Ixodes (deer) tick to transmit more and more Borrelia (lyme disease) to humans. Staphylococcus is responsible for the cases of toxic shock syndrome.

Salmonella and numerous other bacteria including E. coli and Campylobacter are causing food-borne illnesses. More people are "eating out" and extensive traveling (including air flights) have been the sources of increased food-borne illnesses. Poor sanitation has led to a recent outbreak of Vibrio (cholera). Poor disinfection of air-cooling systems has promoted the growth of Legionella (legionnaire's disease). Finally, changes in the virulence of Streptococcus is implicated in the "flesh eating" necrotizing fasciitis.

Immunosuppression is related to the recent outbreaks of Candida and protozoal infections. The outbreaks of Cryptosporidium(cryptosporidiosis) have been related to development of housing and drinking water near watershed ares (especially those near dairy farms). Air travel may increase the chance for transmission of Plasmodium (protozoan that causes malaria); and inadequate control of water supplies is related to outbreaks of Giardia.

The clearing of forests and/or exposure to wildlife is a leading cause of new viral outbreaks. Human intrusion into the viruses ecological niches and areas has been associated with the inhaling of dried rodent urine and feces resulting in death-threatening pnemonia caused by the hantavirus. Virus infected monkeys have been the excitement of recent novels and movies; however, the actual shipment of primates throughout the world is a probable cause for ebola virus outbreaks. Direct contract with infected human corpses is the primary method of transmission of the ebola virus so medical staff must practice all universal precautions. Other viral outbreaks include dengue fever, hepatitis, measles, influenza, rabies, and encephalitis. Reports from the WHO and CDC summarize the emerging diseases and report updates every week.

Other syndromes like SIDS (Sudden Infant Death Syndrome), FAS (Fetal Alcohol Syndrome), Gulf Syndrome, and CFS (Chronic Fatigue Syndrome) are currently emerging and being studied. It is unlikely these symdromes are related to micro-organisms. Diet, stress, exposure to chemicals, and other factors are being researched to help determine the causes of these emerging diseases or syndromes.