The Medical Institute provides the following distinction between STD and STI:
“Although the terms "sexually transmitted disease (STD)" and "sexually transmitted infection (STI)" are often used interchangeably, they are by no means identical. STI is by far the more inclusive term. Sexually transmitted infection (STI) is an important and scientifically valid term because dangerous pathogenic organisms can be present in the human body without causing disease. It is therefore appropriate to refer to this condition as a sexually transmitted infection.
Sexually transmitted diseases (STDs) result from damage caused by a sexually transmitted infection (STI) that has progressed. Although all STDs are preceded by STIs, not all STIs result in the development of STDs. For instance, about 90% of women who are infected with human papillomavirus clear their infections within two years. Only women with persistent infections are at risk for developing the disease – cervical cancer. It is important to remember that it is not necessary to have a disease, or any symptoms at all, in order to be contagious. Many people who are infected with STIs that have not yet progressed to STDs have gone on to infect other people.”1
Medical Institute for Sexual Health, “STD and STI: What is the Difference?” October 2004.