You should treat disease transmission vectors as subsets :<p>air &gt; contact &gt; sharing food&#x2F;toilet facilities &gt; sexual &gt; blood<p>Any disease, say the flue, that can be transmitted by &quot;sharing air&quot; can be transmitted by contact, by sharing food, and sexually and blood transfusion (with very few exceptions)<p>They&#x27;re subsets of one another. Sexually transmitted diseases are mostly that because they can&#x27;t be transmitted any other way (other than sharing blood).<p>In fact a number of &quot;sexually transmitted&quot; diseases can&#x27;t be transmitted sexually at all. But apparently it&#x27;s somewhat common for humans&#x27; sexual organs to develop scratches during sex, resulting in blood mixing. So a number of sexually transmitted diseases cannot in fact be transmitted without exchange of blood. And euhm ... scratches are more common with anal or oral sex (sharp teeth near the relevant parts, I guarantee you there&#x27;s 5 tiny bleeding scratches in your mouth right now, especially right after eating and&#x2F;or stretching muscles and tissues beyond their normal limits ... leads to bleeding)<p>(this is not just true in humans, cats for instance are in fact much worse (due to the hook involved ... women might want to skip looking up what that means), when cats breed, you&#x27;re pretty much guaranteed that you can find blood on the relevant parts. In fact, that is considered one of the signs that it&#x27;s happened. Because scientists are not free to experiment on humans but are (more) free to experiment on cats we actually know more about disease transmission in pets and horses than we do about it in humans)