Evolutionary medicine builds on evolutionary biology and explains why natural selection has left us vulnerable to disease. Unfortunately, several misunderstandings exist in the medical literature about the levels and mechanisms of evolution. Reasons for these problems start from the lack of teaching evolutionary biology in medical schools. A common mistake is to assume that “traits must benefit the species, as otherwise the species would have gone extinct in the past”, confusing evolutionary history (phylogeny) with evolutionary function (fitness). Here we summarize some basic aspects of evolutionary medicine by pointing out: 1. Evolution has no aim. 2. For adaptive evolution to occur, a trait does not have to be beneficial to its carrier throughout its entire life. 3. Not every single individual carrying an adaptive trait needs to have higher than average fitness. 4. Traits do not evolve for the benefit of the species. Using examples from the field of neuroimmunomodulation like sickness behaviour (nervous system), testosterone (hormones), and cytokines (immunity), we show how misconceptions arise from not differentiating between the explanatory categories of phylogeny (evolutionary history) and evolutionary function (fitness).