Yes, absolutely. Some diseases are due to pathogens and such. But those should not be "modern", in the sense that said pathogens might not be exactly new. Maybe you could say anthropogenic diseases such as lung cancer from smoking or esophagus cancer from drinking are modern, though.
What I meant by "modern diseases" in my comment is a group of non-communicable diseases or so-called "diseases of civilization".
"Diet-related" is rather ambiguous, so different lines might be drawn there. As an example, would you say Alzheimer's is a diet-related disease? What about melanoma and/or sunburn?
