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The germ theory of disease

Your immune system protects you from bugs: harmful micro-organisms. These are tiny living organisms that can infect and sicken us. There are many types, including:  viruses, bacteria, fungi, protozoa.  It might surprise you to know that it is a very modern idea that germs – another word for bugs – cause disease.  The germ theory has only been widely accepted since the second half of the 19th century, and is still not accepted by some.

Please allow me an arithmetic deviation. When was the 19th century?  Our calendar starts with Jesus.  A century is 100 years, so we describe the first 100 years after His birth as the first century.  So the 19th century are the years from 1800 to 1899, or the 1800s.  In 2016, we are living in the 21st century: science fiction for me,  growing up in mid-20th century.

Wikipedia tells me that germ theory was originally proposed by Girolamo Fracastoro in the year 1546. (I wonder if there has been adequate credit given to earlier Chinese or Greek thinking.  Most ideas can be found in these classical sources,   confirming the biblical verse that ‘there is nothing new under the sun’. ) The microscope was only developed in the 16th century, and it was not till 1676 that  Antonie van Leeuwenhoek discovered micro-organisms.  He had developed a microscope with up to 300 times magnification and  discovered red blood cells and spermatozoa (or sperm, if you remember where you come from).

The role of micro-organisms, or bugs, in causing disease remained controversial until medical science advanced. As these bugs are too small to be seen, they were not discovered until we had microscopes.  Viruses are much smaller than bacteria, and cannot be seen with a light microscope.  An electron microscope  allows us to see viruses, but we can also see the effect of viruses on the cells they infect on a lab dish.  The first virus was only isolated in 1898, influenza virus in 1933.

The cartoon above shows the key features of the influenza virus: the two proteins that it uses to get into and out of the cell it infects and takes over, preparing billions of copies of itself in the infected cells.  Below, you can see these two proteins as well as the 8 strands of its genetic code, each coding for the proteins that make up the virus.  Influenza is an RNA virus, meaning its code is made of RiboNucleic Acid.  In contrast, bacteria and humans use DNA, or DeoxyRiboNucleic Acid, as our genetic code, in each of our cells.

 

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The influenza virus 

Source: http://www.twiv.tv/virus-structure/  where you can see other types of virus.

Our understanding of the immune system is even more recent than of bugs.  Many different types of viruses cause disease; and even more so for bacteria. For most viruses, including influenza the symptoms we get are not from the infection itself, but from the body’s attempt to remove an invader that has breached the initial defence barriers of skin, gut, or vagina.  This is why many viruses will cause an influenza-like illness.

Other viruses, like measles, polio, or Ebola each have distinctive patterns of illness, that still is the reflection of the battle between the virus and the specific tissues that that virus has a propensity to attack. In addition to an acute infection, some vaccines can be carried for a long time to be reactivated as shingles or cancer. hepatitis B and human papilloma virus cause cancer of the liver and cervical cancer, respectively.

The immune system does not only protect us from these infections, but we can teach the immune system so that it can defeat an infection before it sickens us.  But to understand the immune system, we first need to understand cells….

 

 

 

 

Diversion on dates:

So, was Jesus born in Year zero or one? Probably neither, but in 4BC – which means four years before Christ is born, so how can that be?  The problem was this new calendar, called Annus Domini (AD) , was not established till some hundreds of years later.    The issue of whether to choose zero or one as the starting point is a computer coding question too.  Are you 0-day or 1-day old on the day your are born?  The zero itself was not born till seventh century India, around 650 AD (7th century), though did not reach Europe till the 12th century I hope by now you will recognise is the seventh century after His birth.  And that in 2016, we are living in the 21st century.  Can you believe it?

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Does Logic Work? Anti-Vax arguments to teach thinking…

I have this dream.  An educational resource to teach children to think: numeracy, logic, and how different an issue seems from an individual versus a population perspective.  In part, this is my response to anti-vaccine arguments that flourished following a 1998 Lancet article; but are as old as the first vaccine:

In 1796, Edward Jenner demonstrated that prior ‘vaccination’ with cowpox would lead to protection against smallpox.  Smallpox was one of the worst scourges of humanity, until eradicated in 1977 through a global immunization programme, led by the World Health Organization.

Anti-vaccine arguments date back to Jenner:  like most vaccines smallpox can cause serious adverse reactions, but it does not turn anybody into a cow – as depicted in the first image of the anti-vaccine movement.

I don’t know if anybody seriously believed that smallpox vaccine – cowpox that has been altered through multiple generations of human transmission – could turn one into a cow.  But the list of ills caused by modern vaccines, according to some anti-vaxxers, covers a large range of human illnesses for which we either have not yet defined the cause or there are multiple causes.

In fact, smallpox vaccine has amongst the worst safety profiles of vaccines.  Luckily for humans, this is now only of historical interest.  Smallpox was the first disease eradicated by a vaccine; though some anti-vaxxers can provide entertaining accounts of why it really has not been eradicated, or that its eradication was purely coincidental to the vaccine programme.

In 1999, I was invited to write WHO guidance on adverse events following immunisation.  The AEFI is a carefully crafted concept that requires understanding of the logical fallacy of post hoc, ergo propter hoc.  This is Latin for mis-attributing to coincidence a causal relation: when event A happens after event B, we can say that B caused A.

In reading through the science of AEFI, we classified events that happened after as being possibly, but not necessarily caused, by the vaccine.  The event could be from an error in storing, preparing or giving the vaccine: a programme error.  These remain too common in developing countries, the main audience for the WHO manual.  Injection and anxiety related reactions are relatively common, especially in poorly planned mass campaigns. But coincidental events are the main problem for immunisation programmes are those that just happen to occur after immunisation.  So, why is my claim for an event being coincidental stronger than the claim around smallpox eradication?

Of course, some AEFIs are caused by the vaccine. The known vaccine reactions have been well defined during initial trials; rarer events require wide-scale use in programmes to become evident. During my time leading the NZ immunisation programme, I found it ironic that the vaccine with the most severe known reaction (roughly 1 in a million risk of paralysis) was the one that caused the least public concern about vaccine risks. Perhaps because it was the only oral vaccine in the national immunisation schedule at that time.

In contrast, there is roughly a 1 in 200 risk of paralysis from infection with the poliovirus.  The risk difference between disease and vaccine is why immunisation improves health.  But human cognition is not well designed to compare risks.  For example, we worry more about getting on an airplane than in a car; while on a tropical beach we fear sharks more than a coconut falling on our head. (I hope that you know the first risks are statistically much lower than latter.)

So, you can see that our intuitive risk meter is off; but we can correct this through deliberate process of logical thinking together with careful review of the available evidence.  What I want is an education resource that helps children understand how to think in that deliberate process.  Which involves understanding numbers and logic, as well as the inevitable flaws in all kinds of data.  And the challenge of how to carefully appraise the evidence.  So, what does this education resource look like?

I look forward to contribution, as I explore these ideas, starting with the number one and how 1+1 can equal 0,1,2 or 3 – and the intrinsic ‘slipperiness’ of numbers.