[thedoc (OP)]

enrico lucarelli asked the Naked Scientists:
   
The plague is supposed to have killed 40 percent of the population of Costantinopolis during Giustinian time, I would like to know why 60% of the population was spared? Why the entire population was not wiped out?

Thank you very much for the good show.

Enrico

What do you think?

[evan_au]

The most common theory today is that the black plague was caused by a bacterium Yersinia pestis, which was carried by fleas, riding on rats and accidentally transported by merchants.

There are various ways that victims can be infected, including flea bites (about 80% mortality) and via the lungs (95% mortality).

As with many fatal diseases, it is a race between the victim's immune system, and the lethal pathogen. If the victim manages to mount an immune response before the pathogen severely weakens the victim, they have a chance to survive; this immunity will then protect them against later exposure to the same pathogen. There is a narrow window of opportunity for the immune system to react to a fast-growing pathogen like Yersinia pestis. This is assuming the victim is well-nourished, they don't have other pre-existing conditions that have weakened them, and are not abandoned when they become ill.

The immune system is different between different people - we inherit some components of the immune system from our parents, which our immune system then shuffles to create many immune cells always on the lookout for external invaders. It is possible that some versions of these MHC proteins may have been more effective at detecting this particular pathogen.

It is also possible that some individuals may have been exposed to a weakened form of the bacterium, which did not kill them, but allowed development of an immune response.

Today, infections with this bacterium still occur, but if diagnosed rapidly, they are easily defeated by antibiotics.

[evan_au]

A recent paper in Nature suggests that a mother's vitamin A intake can affect the development of a baby's immune system - the lymphoid tissue. More lymphoid tissue means a potentially faster response to pathogens.