The Plague

In about 4 hours, I’ll be on my way to my 3 week long Vietnam holiday. I apologise in advance for James’ articles. Anyway, this is the last you’ll hear of me for a while. I was too unorganised to write something original this week, so you’re stuck with another old On Dit article.  As per usual, sorry for any factual, grammatical or errors otherwise.

The Black Death

The Plague killed an incredibly significant proportion of people in the middle ages. Approximately 25% – 40% of the population of Europe died during this time (they are approximates because the people who usually did the counting of bodies had died due to the high virulence). It was the first time that human death rate was higher than the birth rate. Blah, blah, blah. I could throw statistics like this at you all day, but they’re boring, a waste of time and you already know that Plague killed a lot of people. This article is about teaching you stuff you don’t know.

There have actually been three recorded pandemics (an epidemic that is spread over continents) of Plague. Number one started in Egypt in 542 AD, spreading to Europe, Africa and Asia. It killed around 50% of the people that lived around there. It hung around until about 600AD. The second was the better known Black Death in Europe. The third (and still ongoing) pandemic started in 1855 in China and spread to Europe and the USA, where around 10 cases are reported each year. Countless epidemics within continents have also occurred with similar mortality rates.

In modern times, the amount of people reported to be infected by plague each year to the WHO is around 2000, out of which around 200 die. These numbers are probably much lower than the actual number of cases because countries are reluctant to proclaim that they are plague-mongers due to the stigma attached to it. In this way, the world may have been duped into a false sense of security.

Of rats and men

Plague is caused by the bacterium Yersinia pestis (named after Alexandre Yersin, who first isolated it in 1894). Humans are an inadvertent host; the natural cycle was from rat to rat transmitted by fleas. However, since evolution teaches us that mice and men are really, really similar (that’s right, we scientists don’t cut them up just because they’re cute; we actually get proper research pertaining to humans out of them due to this similarity), humans could support the bacterium as well.

We’ll start the cycle at the rat/flea interface: flea bites infected rat with Y. pestis in its blood. In taking in a blood meal, the flea inadvertently sucks up some bacterium too. Now, in the bacterium’s frame of reference, it would really suck if it went straight through to the flea’s stomach where it’d get digested and die. So it does something really quite beautiful; it releases an enzyme called coagulase while passing towards the stomach. What this does is form a clot in the flea’s oesophagus. This does three things: stops the bacterium from being digested; allows the bacterium to divide in the clot while stealing supplements from any other blood meals the flea digests; and starves the flea, making it more likely to bite more hosts that it can infect. The effect of the starvation is two-fold since, to start getting yummy bits into its stomach, the flea must first regurgitate the clot before another meal. This ensures that the bacteria will be thrown up into the host when enough bacteria have grown in the flea’s throat. The act of producing one protein to produce these finely tuned benefits is one of the things that really impress me about bacteria.

Let’s say that a human is bitten. The white blood cells see the bacteria as the foreign threats that they are and engulf it. However, Y. pestis has a protein on its surface that allows it to escape digestion by white blood cells. Instead, it fashions a home out of the white blood cell (which has now migrated to a lymph node) and replicates in it. This huge number of bacteria makes the lymph node swell up painfully into a large lump called a bubo (which is where we get the name bubonic plague). This lump can turn black because of bruising. This is the most common type of plague with an untreated mortality rate of 40 – 60%, but a treated mortality rate of 1 – 15%.

Treatment comes in the relatively simple form of antibiotics. There are no vaccines available that are effective against Y. pestis but those at risk are given a prophylactic dose of antibiotics.

If untreated, however, a huge number of bacteria can escape the lymph nodes into the bloodstream. This can cause septic shock. If you’ve read previous or future articles, you will have seen that many other diseases produce death in this way, but I’ve never actually explained it. Well, today’s your lucky day.

In response to foreign particles, there’s an automatic response to allow more blood to the place of invasion. It does this by opening up the blood vessels wider and leakier. You will have seen this when you get a splinter and it gets red and puffy. This local response is a good thing because you get more white blood cells at the site and neutralise any unfriendlies before they get out of hand.

However, if you get massive septicaemia (bacteria in the blood), every part of you says there’s a foreign particle present, so all the blood vessels widen. This is akin to punching a whole lot of holes in a garden hose. Pressure decreases and blood doesn’t get to important organs, such as the kidneys, liver and brain. Sufferers get a graven plaster on their face, receive damage to their organs or have them fail, fall into a coma and die.

Those that aren’t treated for septicaemic Plague die, whereas treatment gives them a chance for survival of 60%.

Bacteria in the blood can then progress to the lungs and cause pneumonic Plague. In this form, Y. pestis can become aerosolised and be transferred from human to human. Patients cough up infectious sputum and blood. If this form isn’t identified and treated within 24 hours, the person will die of either septic shock or pneumonia.

There are worries of bioterrorism using an aerosol version of pneumonic Plague. Having read the symptoms, you will be able to see that it isn’t something to be considered lightly. The fact that it is still around infecting people makes it much more accessible and obtainable than, say, smallpox. However, aerosoling the bacteria in a way that doesn’t disrupt its delicate structure is a complex task. Do feel safe in knowing that countries have a plan ready for every likely bioterrorism threat… I think.


Minnaganti VR, Cunha BA. (2003). Plague. (Website accessed at

Prescott LM, Harley JP, Klein DA. (1999). Plague. Microbiology (4th edition). McGraw Hill. Pgs 777.

Salyers AA, Whitt DD. (2002). Yersinia pestis, the cause of plague, and its relatives. Bacterial Pathogenesis: a molecular approach (2nd edition). USA, Washington. ASM Press. Pages 202 – 15.

World Health Organisation. (2004). Impact of plague. (Website accessed at



Filed under Thomas' Corner

3 responses to “The Plague

  1. You apoligise for MY articles? Pfft
    Enjoy your holiday

  2. Pingback: Crohn’s Disease – Your Body Hates You Guts « Disease of the Week!

  3. Pingback: The Wednesday Post (17/11/2010) | Disease of the Week!

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