A story from New Zealand demonstrates the economic damage that a little faeces in the water supply can do. It highlights two risk factors - our lowered immunity to faecal bacteria, coliforms and pathogens and the extent to which we depend on sources of food and water that can be readily contaminated. There is a third risk - everyone can produce toxic faeces, in large and repetitive quantities. It is the weapon of the weaponless, and the poorer and more ill-conditioned the donor, the greater the toxicity. Anyone who works in a supermarket, in food distribution, manufacturing or packaging, in a restaurant or fast food outlet is in an ideal and undetectable position to distribute faecal contamination effectively - far more effectively than evacuating ones bowels into a reservoir, anyhow.
In my old part of South London two health conditions were permanent and endemic - head lice and norovirus. I avoided both by adopting the same precautions I used in North Africa. However, zero-hours contracts and the use of casual or illegal workers, none of whom get sick pay, is a bad idea when workers infected with norovirus will continue at work and spread the uncomfortable infection. If you've ever been stuck on a tube with someone suddenly afflicted with projectile vomiting or explosive bowel discharge, it's really no joke.
Most infections are accidental rather than deliberate. The prevalence of minute particles of faeces over bus, train and tube grab handles and straps will make you want to wear gloves in Summer. However, there have also been persistent stories about deliberate contamination of food and ring-pull drinks in supermarket stockrooms. So far, as far as I'm aware, there have been no prosecutions, which may indicate that such tales are apocryphal. Or that police are unmotivated to investigate.
Economic terrorism doesn't need an AK47 - just a handful of toxic poo. There's not much anyone at risk can do except take basic precautions and imagine you're in Tangiers.