There are dozens of different variants of malware; many are the
cause of data breaches. When a hacker is able to install a piece of
malware due to an employee clicking a link in an email, or some other
way,
that malware allows them access to the network, and they can take
control.
Once they are in, they can exfiltrate and copy the data and
expose it on the dark web
or sell it to other hackers. The result is a data breach.