The number of sick days taken by UK workers has fallen to its lowest rate in 25 years. According to the Office for National Statistics, the number of sick days the average employee takes each year has halved from 7.2 days in 1993 to 4.1 days in 2017.

It’s tempting to attribute the overall drop to the rise in popularity of gym memberships and healthy eating. And perhaps some of it is down to that. One in seven people in Britain is now a member of a gym – a record high. While eating well on a tight budget is as difficult as ever, there has been something of a healthy eating boom among those with disposable income, powered by Instagram and the cultivation of more diverse culinary habits.

But something else is going on. Back in 2016 I was working in an Amazon warehouse as part of research for my book. At Amazon, a day off sick was punishable with a disciplinary “point”, whether you phoned in to the manager’s office beforehand or not. Rack up too many points, and workers could find themselves facing disciplinary proceedings. Moreover, we were informed by one of Amazon’s managers on the very first day of the job that we would “just have to self-medicate because we [Amazon] need you here”.

In May, the Chartered Institute of Personnel and Development (CIPD) reported a rise in the number of employees going into work sick; the figure had tripled since 2010. In contrast to the capricious and short-sighted policy enforced by Amazon when I was working there in 2016 (Amazon says it no longer gives out disciplinaries to those taking sick days), the CIPD wants companies to do more to discourage what they term “presenteeism”. After all, turning up to work while unwell damages productivity and – more obviously – can spread illness to other staff.

But since the crash of 2008 our economy has been moving away from some of the more employee-centred thinking that prevailed during the pre-crisis years. There are obviously a lot of companies out there that do not punish their staff for taking days off with illness. But the rise of insecure working arrangements like zero-hours contracts has made it harder for some workers to turn down shifts even when they aren’t well enough to do them. The number of people on zero-hours contracts climbed by 100,000 last year alone, and has risen dramatically in the wake of the financial crisis.

When I interviewed zero-hours workers, I was told on several occasions that the lack of guaranteed hours was sometimes used to get workers to perform tasks and do shifts they might not otherwise have done. If they refused, their hours would be “cut right down”, as one care worker told me. This applied to illness too: if you turned down a shift because you were ill, chances were that you wouldn’t be offered it again.

The rise in the number of people employed in the so-called gig economy – those who are self-employed or independent contractors – is likely to have had a similar effect. There are now almost five million self-employed people in the UK. The typical “gig” worker – whether they are an Uber driver, a Deliveroo courier or a delivery driver for a company such as DPD or Amazon – is not entitled to either sick pay or the minimum wage. Even a single day off can prove expensive. Add to this the fact that some delivery firms actually charge their contractors a fee – up to £150 in some instances – if they can’t find someone else to do their shift for them, and it isn’t hard to see why there has been a rise in the number of workers who are reluctant to stay at home.

It’s certainly plausible that we have all suddenly developed stronger immune systems. But considering the wider structural changes taking place in the economy, it’s much more likely to be the strengthening of the boss’s hand that is making it harder for people to pull a sickie, however wretched they might be feeling.

• James Bloodworth is the author of Hired: Six Months Undercover in Low-Wage Britain