New viruses, worms and Trojan horse programs are identified daily. As of October 2014, McAfee and Bitdefender are two companies, of many, that maintain online virus databases. Three of McAfee's top-tracked viruses originated in 2008, 2009 and 2010. Its website has detailed, up-to-date information on the viruses that are current threats and provides instructions on how to remove them.
Computer viruses infect existing programs. This is similar to the way biological viruses need to insert themselves into a live cell to survive and reproduce. Virus writers identify weaknesses in popular software programs, such as word processing and email applications, and then create a customized virus for it. Software companies must publish frequent updates to combat viruses and address vulnerabilities. Viruses are more prevalent in PCs and Android-based devices than on Macs and iPhones.
Worms and Trojans are similar to viruses in that they seek out vulnerable computers and inflict damage once they gain access. Worms are different from viruses in that they are standalone programs that copy themselves onto other devices. Trojans are programs that appear to be helpful, fun or useful programs but actually act as back doors that allow the virus writer unrestricted access to the infected system.