A pandemic is an epidemic (an outbreak of an infectious disease) that spreads worldwide, or at least across a large region.
There have been a number of significant pandemics in human history, generally zoonoses that came about with domestication of animals - such as influenza and tuberculosis.
Lassa fever, Rift Valley fever, Marburg virus, Ebola virus and Bolivian hemorrhagic fever are highly contagious and deadly diseases with the theoretical potential to become pandemics.
Their ability to spread efficiently enough to cause a pandemic is limited, however, as transmission of these viruses requires close contact with the infected vector.
Genetic mutations could occur at which could elevate their potential for causing widespread harm, thus close observation by contagious disease specialists is merited.
In February 2004, avian influenza virus was detected in birds in Vietnam, increasing fears of the emergence of new variant strains.
It is feared that if the avian influenza virus combines with a human influenza virus (in a bird or a human), the new subtype created could be both highly contagious and highly lethal in humans.
Such a subtype could cause a global influenza pandemic, similar to the Spanish Flu, or the lower mortality pandemics such as the Asian Flu and the Hong Kong Flu.