Five of the world's deadliest diseases known to man
The Ebola virus has killed more than 670 people in West Africa recently and is now considered a "threat" to the UK.
Foreign Secretary Philip Hammond has told the BBC the government is viewing the outbreak very seriously.
Ebola is one of the world's most virulent diseases, according to the World Health Organisation (WHO), and up to 90% of sufferers die from the virus.
This latest outbreak was reported in Guinea in February. It has spread to Liberia and Sierra Leone.
Before being eradicated in the 20th century, smallpox was one of the deadliest diseases in human history.
The last known natural case was in Somalia in 1977, according to WHO.
The only known cases since then were caused by an accident in a Birmingham laboratory in 1978, which killed one person and created a limited outbreak.
WHO has a small stockpile of smallpox vaccine, as do other countries and organisations, but there has been concern in recent years that terrorists could get hold of the virus to use as a biological weapon.
Seasonal influenza is thought by WHO to have annual global attack rate estimated at 5%-10% in adults and 20%-30% in children.
This means an there are an estimated three to five million cases of severe illness, and about 250,000 to 500,000 deaths each year.
The most notorious outbreak in modern history was the Spanish Flu epidemic of 1918, which is said to have killed more people than World War One - about a third of the world's population, or 500m people were infected and estimates suggest 50m died.
A strain of avian influenza or bird flu has been spreading mainly through Asia since 2003, with 650 deaths confirmed by WHO.
During the 14th and 17th centuries the Black Death ravaged Europe and Asia.
Infection was spread from large numbers of rats to man through bites from fleas, causing deadly bubonic plague and a highly contagious strain of pneumonia.
In 2010, 17 people were found to have plague in Peru - four were pneumonic plague, 12 were bubonic plague and one was septicemic plague.
Other outbreaks of plague, which can have symptoms including a cough, swollen lymph glands and high fever, have occurred in China, Algeria and the Democratic Republic of Congo in recent years.
In 2003 doctors from WHO became aware of the virus Sars, or severe acute respiratory syndrome.
The virus, first spotted in China, went on to infect more than 8,000 people and kill more than 750.
Sars causes sufferers to run a fever, have flu-like symptoms and have difficulty breathing. There is currently no known cure or vaccine.
Malaria is present in almost 100 countries and threatens half of the world's population, says WHO.
The organisation's latest estimates suggest 207m people were infected with malaria in 2012, with about 627,000 deaths, mostly in children under five.
The disease, transmitted through the bite of mosquitoes, is both preventable and treatable.
People going from the UK to countries where the disease is prevalent are advised to speak to their doctor before they travel.