In its annual World Health Report, the United Nations agency warned there was a good possibility that another major scourge like AIDS, SARS or Ebola fever with the potential of killing millions would appear in the coming years.
"Infectious diseases are now spreading geographically much faster than at any time in history," the WHO said.
It said all countries must share essential health data, such as virus samples and reports of outbreaks, as required under international health rules, to mitigate such risks.
It said it was vital to keep watch for new threats like the emergence in 2003 of SARS, or Severe Acute Respiratory Syndrome, which spread from China to 30 countries and killed 800 people.
"It would be extremely naive and complacent to assume that there will not be another disease like AIDS, another Ebola, or another SARS, sooner or later," the report warned.
Although the H5N1 bird flu virus has not mutated into a form that passes easily between humans, as many scientists had feared, the next influenza pandemic was "likely to be of an avian variety" and could affect some 1.5 billion people.
"The question of a pandemic of influenza from this virus or another avian influenza virus is still a matter of when, not if," the WHO said.
In all there are 39 new pathogens that were unknown a generation ago.
The report called for renewed efforts to monitor, prevent and control epidemic-prone ailments such as cholera, yellow fever and meningococcal diseases.
International assistance may be required to help health workers in poorer countries identify and contain outbreaks of emerging viral diseases such as Ebola and Marburg hemorrhagic fever, the WHO said.
It warned that global efforts to control infectious diseases have already been "seriously jeopardized" by widespread drug resistance, a consequence of poor medical treatment and misuse of antibiotics.
This is a particular problem in tuberculosis, where extensively drug-resistant (XDR-TB) strains of the contagious respiratory ailment have emerged worldwide.
"Drug resistance is also evident in diarrheal diseases, hospital-acquired infections, malaria, meningitis, respiratory tract infections, and sexually transmitted infections, and is emerging in HIV," the report declared.
While the governments of WHO's 193 member states would ideally be the first source of information in any outbreak, that is often not the case. Nearly half of all of WHO's outbreak alerts come from the media and are then followed up by affected countries.
WHO's annual report also urges countries to share viruses to help develop vaccines and to tighten domestic efforts to combat disease outbreaks.
But an ongoing battle with Indonesia, the nation hardest hit by the deadly H5N1 strain of bird flu, has yet to be resolved. Though Indonesia has said it would send human bird flu virus samples to WHO, it has not yet fully shared the samples.
Indonesia has repeatedly demanded assurances that any pandemic vaccines developed would be affordable for developing nations. Instead of sharing viruses with WHO, Indonesia has signed agreements with vaccine makers, promising to share samples in exchange for vaccine expertise.
China stopped sharing H5N1 specimens with WHO for almost a year before finally sending samples in June, while Vietnam said it sent samples but has encountered shipping road blocks.
In 1951, when WHO issued its first set of health regulations to prevent the international spread of diseases, the situation was stable, the report said. People traveled internationally by ship, slowing the spread of diseases around the world and new diseases were rare.
But today, high volumes of people can quickly travel worldwide, meaning an outbreak or epidemic in any part of the planet is only a few hours away from becoming an imminent threat somewhere else, the report said. Over the last five years, WHO confirmed more than 1,100 disease outbreaks worldwide, such as cholera, polio and bird flu.
It further stressed that accidents involving toxic chemicals, nuclear power and other environmental disasters should also be communicated quickly and clearly to minimize public health threats.