The Journal Citation Reports (JCR) of the Science Citation Index (SCI), published by the Institute of Scientific Information (ISI), provides a comprehensive database for analysis of journals. Recent use of JCR's journal impact factor for evaluation of journals and authors has provoked strongly mixed reactions among investigators. This paper examines the effects of the rapidly expanding literature on the impact factor over the past decade and examines the limitations of journal impact factor for evaluating individual author's contributions. The JCR analyzed 6088 journals in 2005, a 32% increase in new listings since 1995. During the same period, there was a 39% increase in new journal listings in the infectious diseases category. The phenomenon of journal proliferation has had a profound effect on the journal impact factor. During the past decade an increased impact factor was observed in 92% of the top ranking major journals, especially in young and rapidly expanding research fields. Certain highly cited new journals published primarily review articles - not original contributions. There was no increase in impact factor among some of the best known journals, such as Journal of Experimental Medicine, Proceedings of the National Academy of Sciences (USA), and Journal of Infectious Diseases. Clearly, journal impact factor is an imperfect tool for measuring the quality of articles, and its use in evaluating authors has inherent risks. In spite of its limitations, journal impact factor can be used as a rough indicator of scientific quality in specific subject categories and for serious reading and learning.