Q&A: H7N9 bird flu
A new type of flu has made the jump from birds to people in China - it is known as H7N9.
The World Health Organization says it as an "unusually dangerous virus", but it is still too soon to say whether it will pose a global threat to human health.
How bad is it?
Scientists have described the new virus as a "serious threat" and a "cause for concern". Since the first case was detected, at the end of March, more than 100 people have been infected - this is an unusually high rate for a new infection. A fifth of the patients have died, a fifth have recovered and the rest remain critically ill. There is no doubt H7N9 is a dangerous infection.
What are the symptoms?
The main symptom is severe pneumonia. The virus can also overload the immune system, causing what is known as cytokine storm. Blood poisoning and organ failure are also possible.
Should I worry?
Not yet. It is a virus that can spread only from a bird to a person, at least for now. It has been detected in chickens, ducks and pigeons at markets in China. Unless you have close contact with birds in the regions affected, the risks are low. However, this can change as the virus mutates. The number of new cases has also slowed with a gap in the middle of May when no new infections were reported for more than two weeks.
What should I keep an eye out for?
The biggest alarm bell with a new bird flu is if it starts to spread among people rather than via birds. The technical phrase is human-to-human transmission. This would give it the capacity to spread around the world in both poor and rich countries. The virus is not there yet, but the concern is that it will mutate.
It will also be a problem if cases continue to soar. This is a numbers game - the more people infected, the more times the virus replicates, the greater the chance of it becoming able to spread in people. Also if the virus jumps to other animals, particularly pigs, this can also increase the chances of dangerous mutations developing.
Has this happened before?
Viruses jump species quite regularly. The last major bird flu, H5N1, made the jump to people in 1997 and has since killed more than 300 people. But H5N1 also remains unable to spread between humans. The most infamous pandemic - the 1918 Spanish flu, which killed perhaps 40 million people - probably came from birds. The fear is that eventually a new virus will sweep the world in a similar fashion. Once again it is simply a numbers game, eventually it will happen and some would argue we are overdue.
How does H7N9 bird flu differ from H5N1?
H7N9 is less deadly than H5N1, which killed 60% of those infected. This is a good thing for people infected, but may be a bigger problem globally as it means more people would be able to spread the infection if it could be passed from person to person.
The other major difference is in the birds. H5N1 killed birds, H7N9 does not. This is important as it is much easier to track the spread of a fatal infection, and H7N9 infected flocks will be harder to detect.
What is a pandemic?
It is when the same strain of an infection is popping up all over the world.
How close to a pandemic are we?
It is impossible to say in weeks or months how far off a pandemic we are. But there is information inside the virus's genetic code which provides some clues. Controversial research to modify the H5N1 virus highlighted five key mutations needed for it to spread between people.
Two of the five have been found in H7N9. It shows the new bird flu has some, but not all, of the mutations which would help if become a pandemic. But that does not mean it will acquire the other three mutations or what will happen if it does.
Where has the virus spread?
No human cases have been detected outside of China.
Is it safe to visit China?
The World Health Organization has not recommended any travel restrictions as the total number of cases so far is still low.
Could there be cases elsewhere?
Yes, the virus could spread through birds to other countries. People could also pick up the infection in China and travel before they start to feel ill. It means countries around the world are preparing for cases.
Do the drugs work?
The H7N9 influenza virus does seem to be susceptible to antiviral drugs such as Tamiflu, but there is little experience of using them in the field. However, a study published in May showed that resistance emerged in three out of 14 people being treated at one hospital. Doctors there raised concerns at the "apparent ease" with which the virus developed resistance to the drug.
Is there a vaccine?
At the moment there is no vaccine, but the earliest stages of developing one are under way. It is not yet known if there will be a need for one. It will be a difficult decision to start manufacturing a vaccine as there is limited global capacity for making influenza vaccines. It is grown in hens' eggs and takes months. An H7N9 vaccine would affect the production of seasonal flu vaccines.
What do those H's and N's mean?
They are used to classify different types of influenza A. The H stands for haemagglutinin and the N stands for neuraminidase. Both are proteins on the surface of the virus which come in different varieties, each of which is given a different number.