When you skate, you sweat blood and tears into your protective gear. When was the last time you washed them?....ew.
After a long skate session, protective gear is damp with sweat and, occasionally, blood. Tossing them into a skate bag, trunk, or other dark areas provides a breeding ground for bacteria. This includes doubling up your gear. Mold will build up over time and destroy the padding, turning it from protection into a hazard. Viruses can spread through shared padding from sneezing or coughing into your elbow pads. (You should never share your protective gear) Exposed road rash to these dirty conditions can cause staph infections and, if left untreated, can kill you.
Checking and cleaning your gear regularly is just as important as tightening your hardware before skating. After your session, hang your padding in a dry and lit area. This will increase the chances of killing bacteria and reducing the foul smell.
Wipe down any plastic parts with a soapy rag. If the shells are removable, take them off to clean them thoroughly. Even with consistent drying and spot cleaning, tiny crevasses can go unnoticed. Deep clean them frequently.
Washing machines can break down your pads. Read the cleaning instructions before tossing them in.
Washing Machine Safe:
Removed the padding from your helmet. Wipe down the hard shell with soapy water, rinse, and let dry. If you can remove hard shells from your knee and elbow pads, do so. Tighten the velcro down as much as possible to prevent it from catching lint. Put the cloth pads in a mesh bag or pillow case to prevent damaging them. (Protect your protective gear!) Add jeans or other non lint producing items into the washer. Use the gentlest setting will the hottest water you can. If your padding smells like something died, pour a cup of white vinegar in with your detergent. If the smell is truly terrible, use the "Not Washing Machine Safe" method in addition to this. 
Not Washing Machine Safe:
If your padding can not be put into a washing machine, fill a tub or bucket with the hottest water you can manage. Add a cup of white vinegar and soak your pads. Massage the pads to extract all unwanted bacteria, viruses, and mold. Swish and soak for 30 min.
DO NOT USE A DYRER.
Pat the gear with a towel and hang dry in a well lit and dry area with lots of air flow. Do not leave them on a hot radiator or heater, it will melt the padding. It takes time to dry (a day or so). Make sure to clean them when they are not needed.