An emerging infectious disease is defined as any disease that has seen new incidences in the past 20 years and seems likely to increase in the future. In most cases, they are caused by a newly strain of a known microorganism or a newly identified microorganism that carries the disease. In other cases, the re-emergence is due to drug resistance of the agent or the interaction with other factors that can lead to its development. The mechanisms of re-emergence are many ranging from microbial adaption as is the case with the Influenza A virus to poverty as is seen in the case of tuberculosis that primarily re-emerges in low-income areas.

A number of emerging infectious diseases are caused by a genetic drift. A gene variant occurs in a population after a certain frequency over time. The changes due to genetic drift are however not driven by adaptive or environmental pressures. This effect is larger in small populations and is felt much lower in large populations. In small populations with a much higher relative frequency, this occurrence can happen in just a few generations. Given time though, the outcome is nearly inevitable for populations of any size.

The Spanish Influenza is the worst infectious disease epidemic in history. Each year brings us closer to another pandemic. The only question is when. According to the journal Emerging Infectious diseases of April 2005, “Optimists once imagined that serious infectious diseases threats would be conquered.” However, the magazine continues to say, “Infectious diseases have continued to emerge and reemerge.” Emerging infectious diseases claim about 15 million lives every year. There is no way of knowing exactly where the next infection will come from. It could be from animals to humans, humans to animals and a reversal to occur.

This is not surprising and emerging infectious diseases like the H5N1 could mutate and develop to the point of becoming another influenza epidemic. First spotted in poultry markets in 1997, the virus kills as many as 80% of those who catch it. The problem with these new emerging infectious diseases is that science may not have an immediate answer to cause of a disease or how it is spread and before long it can wreck havoc. Most of the infectious diseases strike indiscriminately and on a worldwide scale. One question that remains is whether the world will be ready for the next pandemic.