Screening finds people that are likely to have a disease or it looks for factors that are precursors to disease. Mammography, a special X-ray of the breasts, can detect breast cancers early in their development, when they can be easier to treat. Pap smears detect changes in the cervix that can be precursors to cancer. Screening tests can also be used to identify people that are at greater risk of having a disease, for example, cholesterol tests can identify people who are more likely to develop heart disease. In recent years, advances in molecular genetics have resulted in the ability to identify people who are susceptible to developing certain diseases, this will lead to the development of many new kinds of screening tests.

Vivek Goel, Canada

http://www.pitt.edu/~super1/lecture/lec0591/003.htm