Poor ventilation standards is the main reason. You mostly get sick not from yourself just existing, but as a result of being around other humans and animals. At one point in human history, we figured out that re-using drinking water was a bad idea. Now many of us take clean drinking water for granted. Setting up infrastructure so everybody could have clean, safe drinking water all the time was expensive, and it wasn't a one-time expense, it's an ongoing expense. But it was worth it. Nobody I know has gotten typhoid or cholera, and that's a blessing. Unfortunately much of the world still lacks access to clean drinking water.

John Snow is the researcher widely attributed to being the first one to identify re-used water as a source of sickness, he was a pioneer in epidemiology. Vox has an interesting video on it https://www.youtube.com/watch?v=VJ86D_DtyWg

We also learned the same thing about re-using cutlery and plates.

We have let to learn that lesson for air, even though breathing in the wrong breath of air can literally kill you. In some spaces we have learned this lesson (hospitals have excellent ventilation standards), but in most we haven't. Increasing ventilation standards would mean we all get sick less often because we would get exposed to less airborne viruses like flu. Not only does this mean less sickness, but less opportunities for those viruses to replicate, mutate, and potentially become more deadly or immune evasive. Flu is a major underlying aggravating factor that leads to heart attacks due to the stress is places on the body . Which is why the flu shot is the #1 thing you can do (aside from diet and exercise) to prevent a heat attack. CDC estimates flu kills around 28,000 people in the US every year. In prior flu pandemics, of course, like 1918, flu has wiped out 50 million people.

John Snow project is one project advocating for better ventilation standards in public areas like schools. If you're curious about their work, check out http://johnsnowproject.org