Final answer:
Cancer spreads from a primary tumor through mechanisms like local tissue invasion, the lymphatic system, and especially through the bloodstream to distant parts of the body, a process known as metastasis.
Step-by-step explanation:
Cancer cells originally form at a primary site and can aggressively divide and invade nearby tissues, a process known as local spread. Additionally, cancer cells may spread to regional lymph nodes through the lymphatic system. However, the third and most concerning pathway for cancer spread is through the bloodstream to distant sites in a process known as metastasis. As a result, new cancers, termed metastases, form.
Metastasis is associated with advanced stages of cancer and significantly contributes to the difficulty of treatment. Consequently, cancers that have metastasized generally have a poorer prognosis and are responsible for the majority of cancer-related deaths.