Abscesses: To help prevent abscesses, keep your rats and their cages clean and treat any wounds, no matter how small, immediately, keeping in mind that some rats just seem to abscess more than others.
Bumblefoot: Some rats have a genetic predisposition to bumblefoot and some don't. The best way to prevent bumblefoot in those that are predisposed to it is to always keep their cages clean. Never let the bedding go unchanged when soiled and keep any solid shelves free of urine and debris. Keeping your rats at a healthy weight will also help prevent bumblefoot.
Choking/Drooling: To prevent choking, never give your rats extremely sticky foods such as straight peanut butter.
Degloved Tail: Never pick your rat up by the tail. If it is absolutely necessary that you do so, only grasp the tail at the base (near the rat's bottom), never near the end of the tail.
Itching, scabs, bald spots, skin problems: Feed your rat a diet that is low in protein, keep the humidity relatively high in their environment, keep their nails clipped, and always check new rats for parasites before allowing them contact with your existing rats.
Loose stools: Give your rats a probiotic such as Benebac or yogurt with live active cultures during antibiotic treatment. Avoid putting your rats in frightening situations. Don't offer fresh fruits and vegetables as their sole diet.
Mycoplasma Pulmonis: Use a bedding that has very little dust and never use one that contains cedar or pine. Always keep their bedding clean. Do not allow ammonia build-up. Don't expose them to dramatic changes in temperature. Do not house them in a cage that is too small. Feed a well balanced diet and offer them plenty of mental stimulation.
Orange Skin: If you have males with an excess of orange build-up on their backs, brush them on a regular basis and bathe them when they become especially oily.
Ringtail: Keep the humidity in your rats' environment at about 40-55%.
SDA and Sendai: ALWAYS quarantine new rats for at least one month before letting them have ANY contact with your existing rats. Here is some info on successful quarantine: