Quarantining
Quarantining your new rat away from any other ones you have is a very sensible idea, to try and help prevent the spread of any unwanted diseases/illnesses it may be carrying. Rats are extremely prone to all sorts of diseases and illnesses and when you first get a new rat you may not notice any of these unless they are either very far advanced (in which case whoever you got the rat off should have noticed and either prevented it from being sold, or had it treated) or of a physical nature. The quarantine period gives you a chance to see if there are going to be any health problems with the new rat and what, if anything, they may be. This will help prevent you from accidentally passing on anything life-threatening or disabling to the others rats you own.
It must be remembered that the risks of your new rat having anything serious is minimal, as long as you acquire them from a reputable source. However, no matter how small the risk, quarantining is still the best, safest option your have to preventing the spread of anything.
When you buy your new rat, the first step to making sure there are no apparent illnesses would be to make a quick trip to the Vet. Most Vets are certainly more than willing to carry out a small examination of any new rats, as they also know the risks involved in introducing your rats to new illnesses. This won't take very long, and should set your mind at ease just that bit more.
Through the amount of rats I myself have had, as well as friends, we have not once had a case of anything serious from new rats. But this does not mean to say that there will never be anything appearing. Quarantine all new rats. Better safe than sorry!
The average length of time most sources say for a quarantine period is from two weeks to a month. The new rat should be housed in a separate cage and a separate room well away from other rats. This will help prevent any contact diseases from spreading, although air-borne viruses can still be transmitted from one room to another. The only way to completely guarantee that this does not happen is to quarantine your new rats from your old rats in a completely separate building, although for the most part we all know this to be nigh on impossible to do.
To ensure that you are doing all you can, be sure to first wash and disinfect your hands completely after handling the new rat before you even think about handling your other rats. Another good preventative measure would be to change your clothes if you happen to let the new rat on your clothing while handling it before going on to handle your other rats.
Watch the new rat closely for any signs of illness, no matter how trivial they may seem (even sneezing can be the sign of something more sinister). Don't forget to also watch your other rats for signs of illness. If any symptoms appear, be sure to treat immediately.