Physical Danger
The majority of conservationists don’t spend their days chained to a desk; they're out and about in the natural world, oftentimes in foreign countries, surveying the area and doing research or making recommendations.
The most well-known case of a conservationist in danger is the story of Dian Fossey, the zoologist and primatologist who was murdered while studying and living with mountain gorillas of Rwanda. Although Fossey wasn't called a "conservationist," she put a lot of resources and time into preserving the mountain gorilla population.
This case is an extreme example of the tension that has always existed between conservationists and land owners and developers. In this situation, Fossey tangled with the poachers.
It must be noted that people are more dangerous to wildlife than wildlife is to them. However, there have been cases of wildlife conservationists (as well as local people) finding themselves at the business end of a hungry lion. It's not popular to talk about lion conservation in this way, but is a documented fact (though not an everyday occurrence).
Maybe soil is more your thing, where Rhizobia, a single-celled bacterium about one one-thousandth of a meter long, might be your biggest threat…