An insidious hazard is one which is easily overlooked and ignored because they are not usually conspicuous (seen, tasted, smelled, or felt).
People in general have trouble with insidious hazards because there are only two things you can do about an insidious hazard; you can ensure that the hazard isn't present, or you can continuously maintain precautions for the hazard. And it's usually not there, so it involves a lot of effort and attention over nothing. Basic brain wiring tries to insist that this thing that doesn't happen isn't important.
Carbon monoxide is only mostly insidious (you can tell it's there, but you will probably mis-attribute your symptoms); there are laws about carbon monoxide detectors anywhere there's a combustion furnace because of this. Inert gas suffocation (when you walk into a cloud of, say, methane and pass out before you realise what's wrong) is another insidious hazard.
Mercury is an insidious hazard; actual things containing mercury have mostly been dealt with via the first method, by getting rid of the thing with mercury in it. This is only somewhat helpful because while there are sometimes regulations designed to limit the amount of mercury released into the environment by industrial activities like burning coal, these are generally not effective because they're seen as an intolerable limitation on profit. (I suspect this would be true even if it was readily possible to point to specific corpses.)
In other contexts, fixing stuff down manholes or dealing with silage could, in principle, use expensive laser spectroscopy or other detection mechanisms to ensure an inert gas hazard isn't present; one could, in principle, always wear breathing apparatus if there might be an inert gas hazard. In practice, people don't; it's too expensive and it's too slow. The occasional death is seen as preferable to the cost of mitigating the insidious hazard.
COVID-19 is an insidious hazard. You won't notice when you catch it; you won't notice who might spread it, because people are most infectious before they have any symptoms at all.
I find myself thinking that the Ontario government's response to COVID-19 is exactly like their response to air pollution (for most, most of the time, an insidious hazard); yes, this kills a lot of people. Not killing those people would be expensive, in the immediate term, for those presently wealthy. We won't do anything about it.
(Note that this policy position is, from a long term financial perspective, flat wrong; the economic case for not killing those people is simple and obvious and strong. So there's no "grim monetary need" case to be made for the necessity of accepting the deaths. It's nothing but preference.)
Wearing a mask, washing your hands, staying away from other people -- never mind the specific distance, the thing is airborne; "as far as you possibly can, and if that's not far enough, don't" -- are the second response to an insidious hazard; always take the precautions. You don't know and won't know and can't know when you're going to need them, so you always need them. Put the mask on before leaving your dwelling and leave it on until you return. If you can't do that, don't do the thing. (If the employee can't do that, don't you go and make them do the thing. If they work for you, fix it.)
Annoying, inconvenient, unwelcome? All of the above.
Sensibly avoidable, heading into a boreal winter where we're severely uncertain we're going to avoid overlapping super-spreader events?