Even at the Royal Free hospital in London, the lead UK specialist centre for Ebola, doctors have limited options for treating their patients. In the end, survival may depend more on the strength of an individual’s immune system than anything medical science is currently able to do.

ZMapp, a drug made by the small biotech company Mapp Biopharmaceutical in San Diego, attracted enormous attention when it was given experimentally to the first Americans and Europeans to be infected with Ebola. Seven people received it, including the British nurse Will Pooley and two Americans, Kent Brantly, a doctor, and missionary Nancy Writebol, all of whom recovered. Two others, however – a Liberian healthcare worker and a Spanish priest – died. This compassionate use of a drug that had successfully treated monkeys in trials proves nothing at all about its effectiveness in humans.

ZMapp is for now out of the picture. There are no more stocks anywhere in the world. The Food and Drug Administration (FDA) in the United States has promised the company more than $42m (£27.6m) to speed up production and trials, but the drug consists of three monoclonal antibodies that have to be grown in the leaves of genetically modified tobacco plants, which takes months. It is also very expensive, which means that mass production for Africa is unlikely.

Two other drugs have entered trials in west Africa – brincidofovir, made by Chimerix in the US, and favipiravir, made by Fujifilm in Japan. These are both antiviral drugs that have been used against flu. Trials have also begun of the use of blood plasma containing antibodies against Ebola from survivors whose immune systems have fought off the disease. All these trials are in the very early stages, however, and it will take six weeks or more before scientists have any indications as to whether the drugs or the plasma are working.

Pauline Cafferkey, the Scottish nurse who was volunteering in Sierra Leone and is now in a critical condition at the Royal Free , has received blood plasma containing antibodies and also one of these antiviral drugs. Doctors will hope to boost her body’s ability to fight off the virus – none of the drugs can kill or suppress the Ebola virus itself.

When the virus enters the body, it starts to replicate, at first without being detected. Cafferkey was not infectious when she arrived at Heathrow because she did not yet have a temperature indicating a fever – the first sign that viral levels have risen high enough to cause symptoms. It is what follows – the vomiting, diarrhoea and bleeding – that will infect other people if they are unprotected. When Cafferkey first arrived at the Royal Free, she was sitting up in bed, reading and talking and apparently well. But within a few days, the virus in her body had replicated to sufficiently high levels to threaten her survival.

At the Royal Free and similar hi-tech hospitals, an Ebola patient who suffers organ failure from the viral attack can be supported in intensive care, which is not possible in west Africa. Cafferkey is in the best possible place to fight off the virus, she is relatively young at 39, and the infection was diagnosed at an early stage. That is all in her favour. But this virus is aggressive, until recently it was rare, and there are no proven drugs against it, so nobody can be sure for any patient what the outcome will be.