The answer is infection. That's the specific term for when disease-causing pathogens invade and start multiplying inside body tissue.
Contamination is a broader term — it just means pathogens are present somewhere, not necessarily inside tissue. Disinfection is the opposite process entirely, the act of killing pathogens off. Fission is unrelated cell-division vocabulary and has nothing to do with infection control.
This is 1 of the real Sanitation questions in fadejunkie's 299-question bank. See the Sanitation practice questions page for a fuller sample, or try the Sanitation practice quiz and flashcards for free. A one-time $15 lifetime pass unlocks the complete question bank across all 13 tested topics.
More real Sanitation questions: What Illness Does Staph Bacteria Cause?, What Disease Do Diplococci Cause?, Are Disinfectants Pesticides?, Which Organ Does Hepatitis Damage?, and 15 more on the Sanitation practice questions page.
Browse every topic on the Practice Question Hub, or see the Texas Barber State Board Exam Guide for the full exam process — required hours, PSI scheduling, and fees.