More people could mean more diseases spreading around the globe.
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It is estimated that the world population will reach 11 billion by 2100, and one of the consequences of this population would rise would be new challenges in controlling disease outbreaks, scientists say.
The population rise is also likely to boost the increasing rate of new infectious diseases caused by emerging viruses and drug-resistant bacteria -- a trend that has already been observed over the past few decades. Scientists are keeping an eye on new viruses that may cause the next pandemic, and are trying to find new cures for pathogens that have long been present in human history but are developing resistance to available drugs. [What 11 Billion People Mean for Disease Outbreaks]
Here are five closely watched diseases that have caused some of the deadliest or scariest outbreaks and pandemics of the last few decades:
Influenza
From the gruesome flu pandemic that killed an estimated 50 million to 100 million people in 1918, to the 2009 swine flu pandemic that took thousands of lives, different strains of influenza viruses have caused some of the deadliest outbreaks of the past century. This group of pathogens is one of the most closely watched due to its enormous pandemic threat.
In its seasonal epidemics, the virus infects up to 15 percent of the population it hits. Annual epidemics are thought to result in between 3 million and 5 million cases of severe illness and between 250,000 and 500,000 deaths every year around the world, according to the World Health Organization (WHO).
The ever-changing virus has gone through several major mutations in the past century and has been able to repeatedly spread to humans via domestic animals. Significant outbreaks caused by the virus include the Asian flu in 1957 and the Hong Kong flu in 1968, which each caused several million deaths worldwide, and the swine flu in 2009.
SARS
SARS, which stands for severe acute respiratory syndrome, is caused by the SARS coronavirus. It first infected people in late 2002 in China, and within weeks spread to 37 countries through air travel. The virus infected 8,000 people worldwide, about 800 of whom died.
Most patients infected by the SARS coronavirus develop pneumonia. The virus spreads by close contact between individuals, and is thought to be most readily transmitted by droplets produced when an infected person sneezes or coughs and the secretions are inhaled by another person. The disease can also spread when a person touches a surface or object contaminated with infectious droplets. SARS may also be spread more widely through the air.
Scientists have traced the virus back to bats. It is thought that the virus found its way into the human population through livestock markets in China.