They are mainly responsible for ;
1. Redirects on blog spot/blogger or word press sites. That is, once a reader clicks on your site or on the malware code, he/she is redirected to another site that are ad serving sites.
2. Ads running on your site or dashboard that you are not unaware of.
3. Random words included in a post. Once you click on those words, an ad pop up, though you are not responsible for it.
How malware negatively affect your site
1. Diminishes user experience on your site: No site user will want to visit a site that is overridled with ads. Imagine you viewing a site, and before you click with your mouse, you are already getting redirected to an online sale site or a porn site, am sure you will not want to visit the site again.
2. It contravenes Google ad words policies, so your adwords will not run. Google frowns on sites that do not give the best quality experience to users or prompts users to sign up or click on ads. In extreme cases, Google can suspend such sites from Adsense or remove the site completely if it is hosted on blogger.
3.The site continues to lose audience and traffic.
What to do: How to scan and remove malware.
1. Put in your site's url in the form box at sitecheck.sucuri.net and scan for malware on your site.
2. The tool scans your site for possible urls, codes and words that redirects users from your site to other sites. The possible culprit codes are displayed.
Having malware on your site comprises its security, hackers can include spams, phishes giving them complete control over your site. You need to protect your site and remove any malware that can give hackers backdoor passes to enter your site. Assume control.