It is often important to quarantine new animals before introducing them to your existing animals. Quarantine can then prevent the spread of disease, including bacterial infections, viruses, funguses and parasites.
To quarantine successfully you will need a separate airspace due to the risk of airborne viruses such as Sendai or Parvovirus. Quarantine can vary greatly in duration and is dependent on the needs of your existing animals and the source of your new ones. A rescue would need to be extremely careful with a pet that had escaped outside for example, as the risks from this are severe.
During quarantine you must not transfer toys between animals and you must wash your hands thoroughly between handling your pets. For a high-risk scenerio you would ideally have another person quarantine pets at a spearate location.