Thanks for the replies!
I wasn't worried about having 100 ohms -- rather, I was wondering if using a 100 ohm resistor between a 5V power and a pin was a safe way of keeping the arduino safe. I guess I have to use a diode though?
Also, I was surprised because I made a mini program where if a pin read a HIGH input, it would turn an LED on. I discovered that the LED would turn on if even I just touched the 5V output with one hand and the input pin with the other. Apparently an input pin needs >3V to read as HIGH, so I was shocked that I was providing at least 3V through my body. Is that right?