Consider the chemical DiHydrogen MonOxide, usually called DHMO. It’s found in many different cancers, but there’s no proven causal link between its presence and the cancers in which it lurks - so far. The figures are astonishing - DHMO has been found in over 95% of all fatal cervical cancers, and in over 85% of all cancers collected from terminal cancer patients.
Despite this, it is still used as an industrial solvent and coolant, as a fire retardant and suppressant, in the manufacture of biological and chemical weapons, in nuclear power plants - and surprisingly, by elite athletes in some endurance sports.
However, the athletes later find that withdrawal from DHMO can be difficult, and sometimes, fatal. Medically, it is almost always involved in diseases that have sweating, vomiting and diarrhoea as their symptoms.
While it has many industrial uses, it is cheap enough to be casually dumped into the environment, where it has many unwanted side-effects. DHMO is a major contributor to acid rain, and is heavily involved in the Greenhouse Effect. In industry, it can short out electrical circuits, and can reduce the efficiency of your car’s brakes. It is used to help distribute pesticides and herbicides - and long after the pesticides and herbicides may have have degraded away, the DHMO will remain, because it is so stable.
One reason that DHMO can be so dangerous is its chameleon-like ability to not only blend in with the background, but also to change its state. As a solid, it causes severe tissue burns, while in its hot gaseous state, it kills hundreds of people each year. Thousands more die each year by breathing in small quantities of liquid DHMO into their lungs.
… oh wait