What is the most dangerous disease?

Dr Nigel: This is a really interesting question. The word ‘disease’ covers all aspects of illness, from infections like flu caused by influenza virus to illnesses like arthritis or cancer and even things to do with the brain like epilepsy (which causes people to have fits). Simply speaking, ‘disease’ means a body or organ that is not working properly. Disease might be in a human, an animal or even a plant (have you heard of Dutch elm disease, for example? – it is a fungus that kills elm trees).

One way to think about diseases is to decide whether they are caused by infections or not. Some of the really nasty diseases are caused by infections, so I’m going to focus on infections in humans to answer your question about the most dangerous disease out there.

One way to measure the danger caused by diseases is to measure case fatality, or what proportion of people die if they get the infection. Here are some of the more impressive examples I could think of:

Ebola virus occurs in Africa and is transmitted in body fluids. Initially it causes a flu-like illness with fever, muscle ache, stomach cramps, headache, confusion and sometimes coma. In some cases it causes bleeding from the body orifices. Case fatality is around 70%. However, there has never been a sustained outbreak in the world (it probably kills people too quickly!) and it only occurs in Africa, so it is unlikely to be dangerous to people in the UK.

Rabies virus is mainly spread by bites in saliva from infected animals. Rabies causes inflammation of the brain and sometimes makes people scared of water (hydrophobia), violent and manic. Case fatality for rabies is nearly 100%. Luckily for us, rabies is not very good at infecting humans and there is a vaccine to protect people who get bitten (and it is only found rarely in a few species of bats in the UK).

Bubonic plague is caused by a bacteria, called Yersinia pestis, that is transmitted by fleas which are carried around by rats. Bubonic plague used to cause some really nasty symptoms, including swelling of the lymph nodes and gangrene of the fingers, toes and other extremities – the skin turns black and people die within about four days. That is apparently why it was called, ‘Black death’!! Case fatality without treatment is about 60% and many millions of people died with this infection in the middle ages. However, the bacteria is now easily treated with antibiotics and improvements in hygiene mean that it is unlikely to cause disease in the UK.

HIV is spread by body fluid contact and is one of the newest diseases known to man. There is a great internet radio programme called Radiolab all about Patient Zero for HIV (www.radiolab.org/2011/nov/14/) that you might like. HIV is dangerous because it infects and kills immune cells, called CD4 positive T lymphocytes. Over about ten years, the immune system of someone with HIV gradually becomes more diseased until it doesn’t work anymore and can’t protect them from simple infections. When this happens they may die of the disease called AIDS (acquired immunodeficiency syndrome). Case fatality used to be very high in the 1980’s and early 1990’s, but now we have drugs that control HIV and can prevent the development of AIDS.

Another way to measure the danger caused by an infectious disease is to find out how many people it can spread to. Scientists measure transmission potential (how likely an infection is to spread) using a number called Ro (pronounced R-naught). Ro is a measure of the number of new cases to expect from each person who is infected. Below I’ve put down some examples of infections with their estimated Ro numbers:

When influenza virus infections were measured in New Zealand in 2009, Ro = 1.25

• Measles is one of the most easily transmitted infections in the world, you just have to be in the same room as someone to be infected, and Ro = 12 – 18
• Small pox has been eradicated by a vaccine programme, but it used to cause lots of deaths in the world, and Ro = 5 – 7
• Polio is also controlled by vaccines, but it causes paralysis and death when it infects people. Fortunately the last case in the UK was in 1982, and Ro = 5 – 7

Measles is probably the winner here with a Ro of 18. If I had measles, this means I might pass it on to 18 other people before my body’s immune system was able to overcome the measles virus. Of course, what really happens depends on lots of other things, including the people I meet. If I stayed at home and didn’t see anyone while I was unwell then I couldn’t pass the infection on to anyone else (I might be pretty bored though), and if everyone I met had been vaccinated they would all be protected from infection. However, even though measles is very infectious, most people who get measles get a rash and a fever and then get better.

I’m not sure which is the most dangerous disease of all, I’ll let you make your own mind up, but it probably depends on who you are, where you are and who you meet on the way!