Security Verification for Google Search - Why is this annoying captcha appearing?

Sometimes, searching Google might return some unusual results that require you to enter a visual captcha in order to verify that you're a human person and not a computer program. However, some of the users might find this security verification very unpleasant and might decide to abandon the search technology
and look for a simpler alternative. Still, why is this annoying captcha appearing? According to Niels Provos, Anti-Malware Team from Google, this verification appears every once in a while when the Google technology detects unusual search queries entered by the users.

The message sent by the search engine is actually an error that starts with the "We're sorry" message and informs us that the "query looks similar to automated requests from a computer virus or spyware application. To protect our users, we can't process your request right now." However, you have the chance to continue the search process by entering the code shown in a photo. "We apologize for the inconvenience, and hope we'll see you again on Google. To continue searching, please type the characters you see below," the message says.

"Several things can trigger the sorry message. Often it's due to infected computers or DSL routers that proxy search traffic through your network - this may be at home or even at a workplace where one or more computers might be infected. Overly aggressive SEO ranking tools may trigger this message, too. In other cases, we have seen self-propagating worms that use Google search to identify vulnerable web servers on the Internet and then exploit them," the Google official tried to explain the reasons for the image verification request.

Google struggles to improve the security of the search technology but with a so powerful engine that indexes websites from every corner of the world, it's very difficult to maintain a high level of security. However, the Mountain View company already included two types of malware notifications that inform the users about the presence of dangerous content on certain pages.