Improvements in sanitation are obviously important, but that alone is not the cause of the decrease of diseases.

If it was, then all diseases would have decreased at the same time. But if you look at the rates of these diseases over time, you will notice that cases did not permanently decrease until after a vaccine for the particular disease was implemented.

For instance, polio cases dropped drastically after the vaccine was introduced in the 1950s. But chickenpox didn't decrease significantly until the chicken pox vaccine was introduced in the 1990s.

If you look at each disease we currently vaccinate for, you'll see that this trend continues.

And there's a reason why I used the phrase "permanently decrease" above. Epidemics frequently look like roller-coasters when graphed. Cases spike during the worst part of the epidemic, then fall when the epidemic is over. If someone wanted to make it look like diseases were going away before a vaccine was introduced, they would cherry pick a date that ended at the end of an epidemic. It's after vaccines are introduced that epidemics end for a disease.

And if there are increased cases of the disease, they are almost always because of reduced vaccination rates. And don't confuse death rates with case rates. We're always developing new antibiotics and medications that save people from dying of diseases.

I mention these last few things because they're frequently brought up in these arguments. Someone will invariably post some cherry picked dates at the end of an epidemic, or post something about cases spiking recently or changing the topic from case rates to death rates (chickenpox) and such. Hopefully people who respond to this post will read it through before jumping in to post and make my point for me.