Infectious diseases can break out suddenly, almost anywhere in the world, and with devastating impact.
The past week has seen Ebola infecting key medical staff in Sierra Leone, a deadly Middle East virus become airborne and a whole city in China put on lock-down for fear of bubonic plague.
The World Health Organisation (WHO), a UN body that exists to protect and advise the international community about threats such as these, raised concerns in May about the "striking changes in the communicable disease situation".
Dr Margaret Chan, the WHO Director-General, told the World Health Assembly in May that a "surge" in cases of the Middle East Respiratory Syndrome (Mers) and the "setback" for the world's efforts to eradicate polio had sparked fears around the world.
That was before the full extent of this year's Ebola outbreak - now the worst that has ever been recorded - was realised.
Experts at the biennial International Aids Conference in Melbourne, Australia this week spoke about their frustration that efforts to defeat HIV/Aids have been going "backwards", while on Monday 28 July the WHO marks World Hepatitis Day by asking the international community to "think again" about a set of diseases that kills around 1.4 million people every year.
With mass threat viruses dominating the headlines, we take a look at 10 of the worst communicable diseases out there - and what is being done to stop them.
Common Management Taxonomy: