How come the Sims 4 doesn't have diseases and viruses?

It does have them already.

There are diseases in Get to Work. I do not want anymore than that.

Not even just at the hospital. You know how in real life if one person in a household gets sick, it usually travels around? I've had that happen in my Sims game. Sometimes it even loops back, just like real life too.

Usually it's just a moodlet, so it can get lost among the other moodlets and be easy to miss. But especially if you have seasons installed, watch in fall and winter for nausea, feeling itchy, stuffy, or even giggles (it's a symptom of one of the illnesses from Get to Work.) Sometimes they'll get spots or stripes too.

There's a few, but it wouldn't hurt to have more.
https://sims.fandom.com/wiki/Illness#The_Sims_4

I wouldn’t mind having diseases that could cause you sim to get sick and die, as long as they are only effecting the sims you are playing at the time, and there are easy cures for them ( in case you don’t want them to die. Also dieses shouldn’t be real diseases, but sim ones. I know my daughter would love to see diseases you could die from in the game. The deaths that bothers me the most are some emotional deaths like ? laughter and embarrassment…… people don’t really die of those things, and I don’t think they should in the game either. I rarely let my sims tell jokes because of that. I can see sims dying of anger, but that is the only emotional death that I even see as realistic. The rodent death, the asteroid death, are a couple that seem more realistic as long as they don’t happen all the time.

We have enough in Get to Work.

Oh wow. I'll need to research them then.

Now that you mentioned it, I do remember them now in the hospital.

My First Pet has that weird rabies thing.