"Websites or web publishers unknowingly incorporate a corrupted or malicious advertisement into their page. Once the advertisement is in place, and visitors begin clicking on it, their computer can become infected: "the user clicks on the ad to visit the advertised site, and instead is directly infected or redirected to a malicious site. These sites trick users into copying viruses or spyware usually disguised as Flash files, which are very popular on the web." Redirection is often built into online advertising, and this spread of malware is often successful because users expect a redirection to happen when clicking on an advertisement. A redirection that is taking place only needs to be co-opted in order to infect a user's computer.
Malvertising often involves the exploitation of trustworthy companies. Those attempting to spread malware place "clean" advertisements on trustworthy sites first in order to gain a good reputation, then they later "insert a virus or spyware in the code behind the ad, and after a mass virus infection is produced, they remove the virus", thus infecting all visitors of the site during that time period. The identities of those responsible are often hard to trace, making it hard to prevent the attacks or stop them altogether, because the "ad network infrastructure is very complex with many linked connections between ads and click-through destinations."
Look here for more: https://en.wikipedia.org/wiki/Malvertising