As the world scrambles to deal with the worst outbreak of Ebola in history, all eyes are on West Africa. But Ebola isn’t the only virus we have to fear. Nigel Burton reports

IN the streets of towns and villages in Sierra Leone the dead rub shoulders with the dying.

The Ebola virus is racing through communities, wiping out entire families, and, with the country's health service in a state of near collapse, there is no one to help.

Treatment stations have insufficient beds, doctors and nurses don't have the right protection and there is a chronic shortage of drugs. Some clinics don't even have running water. To many people in Sierra Leone, an ebola centre is where you go to die.

Ebola first surfaced in Zaire, now the democratic Republic of Congo, 38 years ago.

A team of scientists who visited the village of Yambuku, at the centre of the outbreak, found a virus which killed with devastating efficiency. They named it Ebola after the nearest river to the village. The local translation is Black River.

And now Ebola is wreaking havoc in Sierra Leone, Liberia and Guinea. Thousands have died and, until the world gets its act together, thousands more will suffer the same fate.

But Ebola isn't unique. There are plenty of other deadly viruses circulating at low levels in jungles and rain forests waiting for a chance to spread. So what is out there - and how afraid should we be?

Ebola

Fear factor: moderate to high risk.

Transmission: blood or other bodily fluids.

Airborne: no.

Mortality rate: 70-90 per cent.

Ebola is a hemorrhagic fever which begins with fever, sore throat, muscle pains and headache. It causes internal and external bleeding.

The virus is widespread in Sierra Leone, Guinea and Liberia.

The United Nations fears there may be as many as 10,000 new cases every week by December 1 unless the world responds with more medical help, equipment and money.

There are five distincty strains of Ebola, each named after regions in Africa: Sudan, Tai Forest, Bundibugyo, Reston and Zaire. The latter is the most dangerous. Unfortunately, it is also the strain currently spreading through west Africa

Grounds for optimism: Ebola is a slow-moving virus and only transmits via body fluids. People with the virus are only infectious when they are showing symptoms. Rigorous precautions and careful containment can slow the spread and stop Ebola. In addition, human trials are taking place with a new vaccine and there are a number of treatments which have shown promise.

Hantavirus

Fear factor: low to moderate risk.

Airbone: possibly.

Mortality rate: 10-36 per cent.

In 1485 a mysterious fever swept through England just before the Battle of Bosworth Field. Death following the onset of symptoms was incredibly swift, often within a few hours. The soldiers who watched their comrades die called it the sweating sickness because it caused the victim to shiver and shake. Scientists believe this mystery plague, which died out in 1551, may have been the origins of Hantavirus.

Today this dangerous bug is spread by rodents - it is carried in rat urine, sliva and faeces - although, more worryingly, there have been reports of human-to-human transmission in South America.

Incubation is between two and four weeks the symtoms are similar to other hemorraghic fevers like Ebola.

Grounds for optimism: sufferers stand a better chance of recovery if they are given medical support. A number of vaccines have been developed but have not yet progressed to clinical trials.

Lassa fever

Fear factor: low

Airbone: no.

Mortality rate: up to 50 per cent during an epidemic

A nurse in Nigeria was the first person to catch Lassa virus, a disease spread by rodents. Nowadays it is endemic to countries like Sierra Leone and Guinea. There are around 500,000 cases a year and 5,000 deaths.

Symptoms include cough, chest pains, encephalitis, siezures, bleeding, vomitting and diahorreah.

Grounds for optimism: victims given the drug Ribavirin have a good chance of survival.

Crimea-Congo fever

Fear factor: low to moderate

Airborne: no.

Mortality rate: 30 per cent

Similar to Ebola and Marburg, Crimea-Congo hemorraghic fever is spread by ticks and, once again, is common in west Africa. Outbreaks are usually caused by handling infected animals.

Causes bleeding, vomiting, kidney failure and respiractory distress.

Ground for optimism: a vaccine exists but question marks exist over its effectiveness. Others are on the drawing board but, like Ebola until recently, outbreaks are so sporadic none have been fully developed.

Marburg

Fear factor: moderate

Airborne: no

Mortality rate: 50 per cent

Identified after a small epidemic in Europe in the 1960s. Marburg (named after the German city of the same name) is yet another hemorrhagic virus carried by fruit bats. Russian scientists examined Marburg during the Cold War and some reports suggest they designed a weaponized variant. At least one accident led to the death of a Soviet researcher.

Grounds for optimism: vaccine trials began in 2009 but, to date, no treatment has been approved for human patients.

MERS

Fear factor: moderate to high

Airborne: yes, limited

Mortality rate: 41 per cent

Similar to SARs, MERs has so far been restricted to countries in the Middle East with one or two cases in Europe and America.

Causes flu-like symptoms and eventual respiratory failure. Believed to be airborne but difficult to transmit. Human-to-human transmission has been limited to people in close contact.

At the moment scientists do not know which animal can transmit MErs to humans, although camels are chief suspects.

Grounds for optimism: no vaccine currently exists although some retroviral drugs may be effective treatments.

Flu

Fear factor: high

Airborne: yes, very infectious

Mortality rate: up to 70 per cent

Typically flu kills 500,000 people a year but in 1918-19 it killed more people than died in the First World War.

Flu germs are easily spread and it is virtually impossible to prevent a pandemic occuring in a matter of months. Bird flu, which emerged in Asia 30 years ago, is the deadliest strain but - so far - it has not evolved into a strain that is easily spread.

However, scientists say this is merely a matter of time and, when it does, the world will face another killer pandemic likely to leave millions of dead bodies in its wake.

Grounds for optimism: it is possible to develop flu vaccines but, because the virus is constantly changing, this cannot be done until a killer flu strain is already at large - and a vaccine takes many months to develop. A worldwide vaccination program would take years.