Contagion

The nature programme shows how one chimpanzee's alarm call is taken up throughout the troop until the trees are ringing with frightened hoots. The narrator says other signals are just as catching.

True. I remember reading bedtime stories to my children with my own eyelids drooping. Just reading the word 'yawn' would cause me to do exactly that. I guess yawns are contagious so that the tribe – apart from the nocturnal adolescents who are destined to keep the fire burning and guard the mouth of the cave – will settle down to sleep together. I don't remember my children yawning over bedtime stories but, for me, parenting young children was the most tiring time of my life.

Now that I think about it, many things are catchy, not all of them natural. If my friend checks her phone I feel almost compelled to check mine. Similarly, an ad for coffee, even though I can't smell the aroma, tempts me to put on the kettle. Ditto for chocolate. But of course, advertisers know that.

And laughter. Laughter's catchy. There's even a business in it: laughter yoga, where forced 'ha ha' sounds eventually become the real thing till the room fills with helpless guffaws.

Unusual disorders seem to be contagious too, especially among teenage girls: temporary things like fainting and headaches and more pernicious ones like anorexia and self-harm. A few years ago there was a wave of tics, spread through the appropriately named TikTok, which perplexed neurologists. Not only were they spread via social media – not contagious exactly but definitely viral – but they also lacked the typical neurological patterns of previous tics.

We're social creatures so for better or worse we share our emotions and with all these means of spreading them, no wonder we are recently plagued by anxiety. And if the chimpanzees can be set off even by a recording, why wouldn't all these things be spread among humans electronically?

So, when someone – naively or maliciously, who would know? - sends fear out into the world, is it any surprise that we are all infected?