Where Terrain Theory goes wrong

There’s a growing movement claiming that germs aren’t really what make you sick. Instead, it argues that if your body is truly healthy, you don’t need to worry about germs like viruses or bacteria. According to this idea – known as terrain theory – your “terrain” (how healthy your body is) is what really determines whether you get sick. Some even go as far as saying vaccines aren’t necessary for those who have “optimized” their health (1,2).

Like many misleading theories, terrain theory contains a grain of truth: your overall health can affect how your body responds to infections. People with weaker immune systems – due to chronic diseases (e.g. diabetes), malnutrition, infections (e.g. HIV), medications, or age – often struggle more to fight off infection (3).

But being healthy doesn’t make you immune. You can be young, strong, and healthy — and still get seriously ill from the flu, COVID, or measles (4-6). Health is helpful – it stacks the odds in your favour. But it doesn’t make you bulletproof. 

Terrain theory is not only misleading, it’s harmful (1,2,7,8). It suggests that all illness is your fault – that if you just ate “right” or followed the perfect regimen, you wouldn’t get sick. This narrative puts more blame and stigma on individuals, ignores structural barriers to health, and erases the role of biology, chance, and context. 

A good diet and regular exercise can support your immune system, but they don’t teach it to recognize and fight off specific pathogens – vaccines do. And while healthy habits take time, consistency, and resources not everyone has access to, vaccines offer protection that’s fast, effective, and free for most Canadians (9-12).

Vaccines, healthcare, clean water, hygiene, and healthy living all matter, but they are not interchangeable. They work in different ways to solve different problems (1,13).