Yuehong Zhang reports in brief in Nature[1] the extent of plagiarism in scientific papers submitted to one journal in China:
Since October 2008, we have detected unoriginal material in a staggering 31% of papers submitted to the Journal of Zhejiang University–Science (692 of 2,233 submissions). The publication, designated as a key academic journal by the National Natural Science Foundation of China, was the first in China to sign up for CrossRef's plagiarism-screening service CrossCheck (Nature 466, 167; 2010).
We are therefore campaigning for authors, researchers and editors to be on the alert for plagiarism and to work against cultural misunderstandings. In ancient China, for example, students were typically encouraged to copy the words of their masters.
I saw a really provocative presentation last fall about the extent of plagiarism in papers from several countries. This Chinese journal is not an extreme case -- in some countries, members of National Academies or government ministers are serial plagiarism offenders. We're not talking about reprinting parts of research papers from the same lab; we're talking about wholesale lifting of papers and results from other scholars, often in the U.S, Canada or Europe. The software that enables checking for plagiarism today has revealed a shocking extent of copying in the scientific literature.
The most common explanation is "cultural misunderstanding". This is no doubt true to some extent, but some of the worst offenders are scientists who are anything but naive. There's some shady business out there.