A virus new to humanity, or an old acquaintance dressed up in slightly new clothes, the disease that so far appears to have achieved a 100% kill rate in Johannesburg, is now up against the full might of humanity's scientific detectives.

So far three people have died from the mysterious disease, which has retained its anonymity through an extensive first round of laboratory testing for viral haemorrhagic fevers.

Based on the pattern of disease and a suspected tick bite on the first patient, the pathogen -- let us call it Virus X -- is suspected of being Crimean-Congo Haemorrhagic Fever (CCHF) or Lassa fever, or one of their close relatives.

Such a diagnosis would be relatively good news if Virus X has infected other people, because these pathogens are estimated to kill about 30% of their victims. The nastier haemorrhagic fevers such as Ebola and Marburg have achieved kill rates of up to 90% in some outbreaks. There is also one antiviral, ribavirin, which can be used against CCHF and Lassa.

In the words of one World Health Organisation expert, haemorrhagic viruses kill by "melting the organs". They disrupt the body's internal structure and damage the clotting mechanism. The patient effectively bleeds to death either internally, or with dramatic bleeding from all orifices detailed in several books and films. Blood pressure falls and the patient dies from multi-organ failure.

Professor Guy Richards is an intensive care specialist at Charlotte Mxenge Johannesburg Hospital and one of the doctors who treated the outbreak of Ebola in Johannesburg in the late 1990s. In that case the original patient, a man flown in from Congo, survived. Sadly the nurse treating him died.

He says the haemorrhagic fevers typically attack the endothelial cells lining the blood vessels. The blood vessel system, with its myriad branching arteries, veins and capillaries, is the largest organ inside the body. The membranes of these cells are damaged and fluids leak in and out of the holed cells which are no longer able to maintain their cohesion with their neighbours. Blood vessels collapse, blood pressure falls and the patient dies.

Usually the best medical workers can do is try to keep the patient alive by giving fluids and keeping them cool, and hope that the individual's immune system will be able to fight off the pathogen. Another infectious disease doctor says that the problem is that measures to help the patient, such as inserting drips or tubes to help them breathe, can then trigger further bleeding.

Samples of Virus X are undergoing further tests at the National Institute of Communicable Diseases in Johannesburg, which is one of the few laboratories in any developing country with the infrastructure and skills to test for such dangerous and rare pathogens. The institute's Lucille Blumberg, who leads the laboratory investigations, says that it will probably take several days for Virus X to be identified -- and there is a chance it could be a new virus making its way on to the world stage.

Initial tests for the genetic material of these haemorrhagic viruses, and for the antibodies that victims make against the pathogens, all came back negative. However, there are a range of reasons for this: the individual may have been too sick to produce sufficient antibodies; the levels of virus may have been too small when the samples were taken; the virus may have been a known pathogen which has mutated slightly and so not registered on the highly sensitive and targeted tests used; or it could be a previously unknown virus.

Samples have been flown to the Centers for Disease Control in the United States which collects pathogens from around the world and is probably the world's most important repository of information on infectious diseases. It will have a larger library of different types of viral haemorrhagic fevers against which to compare the South African samples.

The first patient to die as a result of this virus, Cecilia van Deventer, was medi-evacuated from Zambia to Morningside Clinic on September 12 with suspected tick-bite fever. The critically ill woman died shortly after arriving at the hospital. One of the paramedics who had accompanied and helped treat her fell sick as well. He was also flown from Zambia to Johannesburg where he died on September 30. The previous day doctors had linked the two patients, realising they had an outbreak of disease and alerted the national and provincial departments of health. An intensive care nurse who had treated the first patient also sickened and appears to have died from the virus.

An outbreak of dangerous infectious diseases such as haemorrhagic fevers triggers an automatic response: a higher level of infection control to prevent further infections of healthcare workers and tracing everyone who might have caught the virus.

Mediclinic has identified 55 potential contacts, mainly healthcare workers and their families. The department of health has identified another 61. Monitoring these individuals is simple: twice daily temperature checks for 21 days to detect the onset of a fever. The period is designed to cover the usual incubation period of haemorrhagic viral fevers.

Viral haemorrhagic fevers (VHFs) are diseases mainly caused by four families of viruses. They are virus zoonoses -- pathogens which depend on animals or insects for replication and survival.

Zoonoses cause extreme reactions because the immune system sees them as entirely different from other intruders.

The most dangerous VHFs are classified as biosafety level 4, which should be studied only in specially equipped laboratories.

Two other viruses, dengue haemorrhagic fever and yellow fever, can also causes haemorrhagic fevers, although neither is considered such a dangerous disease.

Humans become infected from contact with host animals -- such as a tick bite or contact with the body or fluids of an infected animal. Viruses can be transmitted from person to person through bodily fluids or needles contaminated by blood, faeces, urine, vomit or even semen.

Ebola, Marburg, Lassa and Crimean-Congo haemorrhagic fever can be spread among humans. Small mammals, such as bats, are often the hosts for VHFs, but in Marburg and Ebola the host has not been conclusively identified.

Vaccines exist only against yellow fever and Argentine haemorrhagic fever, although research is under way into vaccines against other VHFs -- especially those which could be "weaponised" for biological warfare.

Marburg virus affects humans and other primates. It is closely related to the Ebola virus, the only other member of the filovirus family. These two are considered the deadliest viruses known to man. Marburg was first recognised in 1967 with simultaneous outbreaks in Germany and former Yugoslavia. Ebola, named after the river in the DRC where it was first recognised in 1976, is often fatal. There are four known types of this virus.

Lassa fever was discovered in Nigeria in 1969. It is endemic in parts of Africa where it causes significant levels of mortality and morbidity.

Crimean-Congo haemorrhagic fever is a tick-borne disease first described in Crimea in 1944 and then recognised in 1969 in Congo. Humans contract the disease from tick bites or through contact with the blood of infected animals, such as cattle or goats. Among hospitalised patients fatality rates vary between 9% and 50%.