There are many important principles in epidemiology, and without more context its impossible to directly answer this question.
One fundamental principle that underscores all of epidemiology is DISEASE DOES NOT OCCUR AT RANDOM.
This may seem obvious, but it was hotly debated for centuries in medicine, science, epidemiology/public health, and religion.
Think of the association of smoking and Lung Cancer. Yes, some people who never smoke will develop lung cancer. And some people who smoke for decades will live into their 80s or 90s, never develop lung cancer, and then die from something else. However, we know that at the population-level, those who smoke are at a higher risk for disease than non-smokers.
This is a fundamental principle in Epi because it means that we can study the cause and risk factors for disease, as well as find targets for cures and prevention. Put another way, if diseases occurred in a TRULY RANDOM way, there would be no point in studying their natural history, causation, or prevention, and therefore, there would be no point in having the field of epidemiology.