Chapter 23: Pathogens and Infection

Hallmarks of cancer include self-sufficiency in growth signals, insensitivity to anti-growth signals, evasion of apoptosis, limitless replicative potential, sustained angiogenesis, and the ability to invade and metastasize. The chapter categorizes genes involved in cancer into oncogenes—mutated forms of normal proto-oncogenes that promote proliferation—and tumor suppressor genes, which normally restrain growth and induce apoptosis. Key oncogenes such as Ras, Myc, and Bcr-Abl, and tumor suppressors like p53, Rb, and APC, are explored for their roles in driving the cancer phenotype. The chapter explains how chromosomal instability, aneuploidy, and mutations in DNA repair pathways (e.g., BRCA1/2) contribute to the progression of malignancy. Epigenetic alterations, including DNA methylation and histone modification, also play a role in silencing tumor suppressors. Cancer development is described as a multi-step process with selective pressures that favor increasingly aggressive clones—a concept known as tumor evolution. The tumor microenvironment, composed of stromal cells, immune cells, and blood vessels, is shown to influence tumor growth and immune evasion. The chapter also addresses how tumors co-opt angiogenesis via VEGF signaling and spread through epithelial-mesenchymal transition (EMT) and metastasis. Finally, the chapter discusses cancer therapies, including chemotherapy, targeted therapies (e.g., kinase inhibitors), immunotherapy (e.g., checkpoint inhibitors), and emerging strategies like CAR-T cells and cancer vaccines. Cancer is framed as a disease of systems failure—requiring a breakdown of regulatory circuits that normally ensure orderly cell behavior.