It’s that strange time of year when, despite springtime breaking out all over town – tulips, apple blossom, sunshine, the works – everyone in the known universe appears to be ill.
I myself have been bouncing back and forth between gastroenteritis and colds for the past month. Despite being a microbiologist by training, I don’t presume any special knowledge about my own illnesses. Or maybe it’s because of my training. Lots of people though, I’m coming to realise, have very strong opinions about infectious illnesses. Last week, my teenage relative, who’d stayed home from school with a debilitating cough just the day before, bent down to plant a big juicy kiss on my baby’s face before I could stop her.
“Don’t worry, I’m no longer contagious,” she told me breezily.
Quite understandably, once our symptoms have peaked and we’re feeling a bit better, we may assume that the danger of spreading illness is past. But is it really that simple? It all depends on the particular microbe and strain. Rhinovirus, for example, one of the agents that causes the common cold, loads up your snot with so many particles that you can easily be infectious a full two days after your symptoms hit, and possibly longer. Other bugs have different windows of contagion, and even considering the same agent, strains can exist with more behavioural diversity. As we rarely get a positive identification of the microbes that inflict us, making assumptions about our own levels of contagiousness is not such a good idea.
Another assumption that doesn’t stand up to scrutiny is some people’s almost mystical certainty about the exact source of the offending microbe. How often do we hear people claiming confidently, “I caught this cold from my colleague at work,” or, much more vindictively (petulance masked as reassurance), “It’s OK to be around me –I actually caught this from you.”
There are untold numbers of contagious microbe strains on the prowl, each with their own incredibly complex infection processes. Because of the variability of incubation periods and viral shedding windows (which can range for days), the most obvious culprit – your spouse, your child at nursery, the chap who sneezed all over you on the Victoria line – may be a red herring. And in a city of eight million, like London, you will be exposed to an astonishing array of people. Transport for London estimates that an average of 24m journeys take place across its network each day, including travellers from other cities and countries making connections. On an average commute to work on London Underground, for example, you might come closer than you’d ideally like to hundreds of people in the carriages and tunnels and platforms; that handrail or escalator rail you are clutching could have been sweated on by thousands before you, many of whom have probably wiped their runny noses with a stray hand.
Unless you are confined at home with no regular visitors, pinpointing the exact moment of infection would be impossible; determining if your strain is the same as your work colleague’s would require complex sequencing procedures well beyond your pay grade.
The assumption about who infected you also leads to a false sense of security about possible outcomes. That same teenaged relative told me a few days later, of my latest stomach complaint, “You’ve got the same thing I had last week. Mine only lasted 24 hours, so you’ll feel much better by tomorrow.”
As much as I’d like to be comforted by such cheerful adolescent omniscience, even if we could be certain we had the same lurgy (see above), differences in our own genetic backgrounds, overall constitutions and previous exposure-based immunities can make the course of the same disease quite different in two different individuals. Germs that only cause frank illness in immunocompromised people are an extreme example of this principle. But each of us is unique, and even otherwise healthy people do not all respond in the same predicable way to an identical microbial onslaught.
I am sometimes asked whether having an acute knowledge of disease transmission is a burden. I used to think so: when I was taking intensive microbiology courses at university and graduate school, I was so freaked out by germs that I was afraid to take a sip of beer from someone else’s glass, or eat leftovers no matter how robustly refrigerated. Now, I’m much more relaxed. The realisation that it’s almost impossible to pinpoint exactly when and where the next nasty microbe will strike brings a sort of liberating release: you can’t know, you can’t run – and you certainly can’t hide.