As per Google a warning appears at the top of the search results page when the computer you’re using is infected with malicious software, also known as “malware” or malicious software. Malware can be used to intercept your computer’s connection to Google and other sites. When Google’s system detects that a connection has been intercepted, it’s likely that the computer was previously infected with malicious software.

An infected computer can result in deleted data, stolen personal information, and a slower connection to many websites.

Why does Google think my computer is infected?

Some forms of malicious software will alter your computer settings to redirect some or all of your traffic through a proxy controlled by the attacker. When you use Google, the proxy forwards your query to the real Google servers to fetch the search results. If our system detects that a search came through one of these proxies, we display the warning.

The Internet brings remarkable benefits to society. Unfortunately, some people use it for harm and their own gain at the expense of others. We believe in the power of the web and information, and we work every day to detect potential abuse of our services and ward off attacks.

As per Google the malware appears to have gotten onto users’ computers from one of roughly a hundred variants of fake antivirus software that has been in circulation for a while which is why no one is aware of a common name for the malware.
As per some statistics about a couple million machines are affected by this malware.

Google addressing to user comments said that they have heard from a number of you that you’re thinking about the potential for an attacker to copy our notice and attempt to point users to a dangerous site instead. It’s a good security practice to be cautious about the links you click, so the spirit of those comments is spot-on. Google thought about this, too, which is why the notice appears only at the top of our search results page. Falsifying the message on this page would require prior compromise of that computer, so the notice is not a risk to additional users.

In the meantime, Google’ve been able to successfully warn hundreds of thousands of users that their computer is infected. These are people who otherwise may never have known.

Source: Google Security