Our immune systems

I've heard before that a "common cold" could have been the death of you in the 1800s and earlier. Therefore, I'd imagine that getting sick wasn't very prevalent. Why is it that nowadays people get sick several times a year? For us, its just an annoying few days until it passes. Most of us continue with our daily routines despite being sick. Thanks to modern medicine and vaccines, our bodies/immune systems have the ability to fight off all different kinds of viruses and what not. Why is it that we still get sick several times a year? How can our bodies be so strong to fight off these crazy diseases such as polio, yellow fever, influenza, yet are still such bitches that we get 3 colds a year? Could it be because most people look to take medicine as soon as they start feeling symptoms and they dont let their bodies fight it off naturally, therefore weakening the bodies ability to fight it off? Someone hit me with some answers, or just speculation.

A) The common cold didn't kill people. Influenza killed people, and it still does. Flu vaccines help and modern practices help but still many flu deaths worldwide.

B) Our bodies fight virii that we have antibodies for; either thru exposure or vaccination. It's not like we are generally better at fighting off *any* virus over the last 200 years. That's not how it works. We've just gotten better at symptom relief and practices that reduce the spread (washing hands).

C) Over the counter medicine has little to no effect on the immune system. It's strictly symptom relief. The exception might be zinc, which can shorten the duration of a cold. But your body is still working hard to fight off the infection. Medicine doesn't 'weaken' your immune system.

D) There's no vaccine for the common cold because there are over 250 virii that cause it, making it too hard (today) for them to create one.