Biosafety Level Four laboratories are used when scientists work with very dangerous or very infectious diseases, such as ebola and other hemorrhagic fevers.

Very strict methods are developed to make sure no part of your skin EVER touches any infectious agent. Just to be sure, scientists working in Level Four laboratories take showers after leaving the laboratory. Every part of your body that might contact a sample of the disease or virus you are working with is protected with protective clothing or some sort of disposable barrier. If the virus or disease you are working with is NOT contained within special air-tight chambers, you yourself are encased in an air-tight suit, with your air supply coming from an air-tight hose connected to a life-support system, sort of like a scuba diver. This suit is disinfected with a chemical shower before you leave the laboratory. If you don't need to wear the air-tight suit, you wear special laboratory clothing (including underwear, pants, a shirt, shoes and a lab coat) that is never removed from the laboratory except for cleaning. Clothes are decontaminated before being cleaned. You wear gloves on your hands (often two or more pairs) that are thrown away after use, and you protect your shoes with plastic "booties" that are also thrown away after each use, so that your shoes never touch the floor. You also wear eye protection (plastic goggles or safety glasses) and a mask over your mouth, like surgeons wear. Most importantly, air-tight suit or not, you never touch the contaminated material directly, but instead handle it with tools that are either cleaned or thrown away.

The entire laboratory where you work is contained within another laboratory, and as you pass from the inner laboratory, where you were working with the disease, to the outer laboratory, which should be kept free of the disease, you pass through a vacuum air lock that prevents any air from the inside laboratory from escaping. The clean space between the two laboratories has changing rooms that allow you to remove used clothes and throw away used material that might be infected, and shower rooms, that allow you to take a shower before putting on clean clothes. All of the disposable material or anything that might be contaminated is passed out of the laboratory by way of a double-doored chamber, which is decontaminated between each use. Material is brought into the laboratory in the same way.

I couldn't find any information about what happens if you do have an accident (the suit or your gloves rip and you spill something on your skin), but if I had to guess I'd say you would soak the area for 20 minutes in disinfectant and then take a shower. You'd probably have to be quarantined and monitored for signs of disease for several weeks, depending on the disease.

This information was taken from the National Institute of Health and the Center for Disease Control, which monitor the use of all infectious material for research.