What is malware?

Malware is a broad term covering many kinds of harmful code. Computer viruses attach themselves to other files and spread when those files are shared, while worms copy themselves across networks automatically. Ransomware encrypts a victim files and demands payment for the key, and spyware secretly records activity such as passwords. Trojans disguise themselves as useful programs to trick people into installing them. One of the costliest outbreaks, the NotPetya attack of 2017, caused an estimated 10 billion dollars in damage worldwide.