Mosquitoes spread diseases such as malaria
New killer diseases are emerging faster than ever across the world, says the World Health Organization.
In its report, "A Safer Future", experts highlighted several major threats to our health in the 21st century. But what are they?
One of the biggest worries for those looking at global health is the sudden rise of a new and deadly illness.
Air travel and a worldwide market in live animals means that a virus could sweep across continents in just a few months.
In 2003, the SARS virus caused an unprecedented panic: A contagious illness, with a week's delay before the emergence of symptoms, and a very high death rate - potentially a very dangerous cocktail.
One of the first known cases, a 72-year-old man from Beijing, took at least one plane trip before the symptoms took hold.
It is thought he infected another 22 people en route, creating a nightmare for experts trying to track down and quarantine those carrying the virus.
Fresh cases were found in Hong Kong, China, and in Canada.
Fortunately for the world, SARS proved to be less infectious than first feared, and the virus was contained within just a few months, not before killing several hundred people.
Scientists believe that an outbreak of pandemic influenza, a new strain jumping from animals into humans, could be a far greater threat, and governments across the world have been asked to plan for its arrival.
Normal seasonal influenza in the UK can kill, but generally only those who are weakened by age or other illness.
A completely new strain of pandemic flu, however, is likely to be far more virulent because humans do not have any resistance to it.
The focus in recent years has been the H5N1 virus in birds, which can infect - and kill - humans, but only those in close contact with affected poultry.
While at the moment, the virus cannot be spread from human to human - the fear is that it will mutate and achieve this.
If H5N1, or any other avian flu virus strain can do this, even with a low death rate compared with SARS, the death toll is likely to be at least in the hundreds of thousands, if not the millions.
In the 1990s, these were the nightmare illnesses, predicted by some to be capable of spreading like wildfire due to modern travel habits.
Viruses such as Marburg and Ebola have some of the highest fatality rates of all, and can kill within just a few days.
Tropical Africa is the hotbed for haemorrhagic fevers, and victims develop a high temperature, diarrhoea, and then severe bleeding, and are highly contagious.
In the 21st Century, an outbreak in Angola claimed more than 200 lives, with nine out of ten of those diagnosed with the illness dying.
The WHO blamed the long civil war in Angola, and its effect on health and transport systems, for the failure to stop its spread quickly.
Ironically, it is the swift and deadly nature of these infections which offers some protection for the rest of the world, as infected people quickly become too ill to travel.
COMEBACKS FROM OLD FOES
It is not just the threat of new diseases that is worrying the WHO. Many of the world's best known diseases have been given a fresh "licence to kill".
A million people die from malaria every year worldwide, and the WHO says that not enough is being spent to stop this number increasing.
This means that malaria is emerging in new areas, or coming back in areas where it was thought to be eradicated.
By the 1960s, the use of insecticides against the mosquitoes spreading the illness meant that, outside sub-Saharan Africa, it was no longer considered a major public health threat.
However, this led to dwindling investment in malaria control.
In addition to this, the parasite that causes the disease is becoming more resistant to some of the most common treatments.
Cholera has made a comeback in the last 25 years, says the WHO, which wants to see renewed efforts to control it.
War, conflict and natural disaster all play their part in its return, as poor sanitation and unclean drinking water are the root cause of outbreaks.
In the aftermath of the Rwandan crisis of 1994, up to 800,000 people crossed the border to refugee camps near the city of Goma in DR Congo.
In the first month after their arrival, an estimated 50,000 died from cholera and dysentery, as Vibrio cholerae contaminated Lake Kivu, the only source of drinking water.
The WHO says that the increasing urbanisation of many countries means thousands living in poor accommodation on the outskirts of major cities, with cholera the inevitable result.
Tuberculosis, one of the major killers of people with AIDS, accounts for approximately 1.5 million deaths worldwide a year.
Antibiotic therapies do exist, although many patients do not have access to them.
A major concern for the WHO is the increasing resistance of the bacterium which causes TB to these antibiotics.