As you know a virus will normally infect an executable file and it will be executed when the infected file is executed. According to the mode of infection the viruses are divided into resident and non-resident viruses. The non resident virus has a module to find the files that it can infect and it also has another module called replication module which will infect the file encountered by the finding module. After infecting a particular file the virus will be executed when the infected file is executed.
In the case of the resident virus, the thing is different. They first infect a file or executed by some other means. When they are executed it loads its replication module into the memory. By working in the memory it is capable of infecting the files to a great extend. So there are two types of resident viruses- those which are capable of infecting large number of files in a short duration called fast infectors and the other that infects less number of files. The fast infecting type virus is somewhat more dangerous since it infect more potential programs in a short duration. If the infected potential files include the files of the antivirus then there is a chance of infecting the files scanned by the antivirus. The fast infecting virus shows the symptoms of infection very soon, mostly by slowing down the PC. There are antiviruses that will be active when there is an abnormality is identified and it will disinfect the infected file. In the case of slow infectors, they do not show the symptoms of infection as slowing downing the PC. This makes them less chance to be identified by the antivirus. But do not remain unidentified forever. Since it shows the signs of infection very late, they are identified very late. However it is less dangerous than that of the fast infectors.