A computer worm is a type of malware that can self-replicate and spread from one computer to another without human intervention. It often uses a computer network to spread itself, relying on security failures on the target computer to access it. Worms can be transmitted via software vulnerabilities, email attachments, instant messages, or spam emails. Once a worm breaches a computers defenses, it can perform several malicious actions such as dropping other malware, consuming bandwidth, deleting files, overloading networks, stealing data, and opening a backdoor. Worms can also modify and delete files, and they can even inject additional malicious software onto a computer. Sometimes a worms primary mission is to replicate itself over and over, depleting system resources such as hard drive space or bandwidth by overloading a shared network. Worms can also steal sensitive data and pave a way for a hacker to get into the computer by installing a backdoor they can access.