Infection transmission

 The COVID-19 pandemic should have got us all thinking about infection transmission and control.  Yet, the virus continues to spread.

I remember the pretty much the first laboratory class we did in Microbiology 101 was a demonstration of how infection transfers within a community.  There were 24 students in the class.

First, everyone wiped their hands with a paper tissue, one of which had been contaminated with a bacterial culture.  This organism was Serratia marcescens, which produces a bright red pigment when grown under the right conditions.  A sequence grid was set up, so that everyone shook hands with one other class member and then plated their hand onto an agar plate.

The second round of hand shakes was then undertaken with different class members, without first washing our hands, and, again, hands were plated.  This sequence continued until, by the end, everyone had shaken hands with about 10 classmates and plated their hands.

The plates were incubated, and a couple of days later were examined for the presence of the bright red colonies.  Scoring the positives against the sequence grid, it was very obvious how the infection had spread throughout the class.  Not everyone became infected initially, but the infection spread exponentially and after six rounds, everyone in the class was positive.

How is this relevant to COVID-19 spread?  This virus is supposedly spread predominantly by droplet inhalation.  However, an individual may become infected by touching their face with infected hands or perhaps by rubbing their eyes.  The virus, like the Serratia, might be picked up on the hands from surfaces like door handles, wash basin surrounds, pens, or from shaking hands with a carrier etc.  "OK, I don't touch my face and eyes".  Are you sure?  I used to lecture to classes of up to 130 students and occasionally counted how many were touching their face at any one time.  (Yes, I know, I had put half of them to sleep and their heads were in their hands).  Up to 60% of the class touched their faces and noses and eyes.  A similar surreptitious survey found that up to 30% of students left the toilets without washing their hands!

The take-home message here is - "Wash and sanitise your hands regularly - many times a day; avoid hand shaking and kissing 😞 and if you are sick, STAY HOME.  In fact, if you don't have to mix with people, stay away from them."  Like in our first year experiment, there was no indication that any particular person was carrying the infection until two days later when the test results were available.

Update:  As of today, 10th July 2021, we have seen over 4 million people die of COVID-19 around the world and it is now obvious that the primary route of transmission is by inhalation of virus particles.  Mask wearing significantly reduces the spread of the virus by trapping droplets and by wearing a mask, you are protecting others.  In countries where people still gather together in crowds, the virus is spreading very fast and the delta variant, which is highly infectious, is ripping through the population.

If you are reading this update and you have the option, please GET VACCINATED, both for your own safety and the protection of your friends and family.