Primary, secondary and tertiary preventions

Primary is when one intervenes before any disease sets in. For example Getting the hepatitis B vaccine at birth is primary prevention.

Secondary prevention is finding and treating a disease before symptoms develop. For example Pap smears to detect cervical cancer and mammographic screening to detect breast cancer.

Tertiary prevention is to prevent complications after the disease has manifested. For example Tamoxifen prophylaxis in a patient who was treated for breast cancer.