105. Read the following passage, then answer the question below.

Long before the word vaccine existed, people had noticed that survivors of smallpox never caught it again. In China, India, and parts of Africa and the Ottoman Empire, healers practiced variolation, deliberately infecting a healthy person with material from a mild smallpox sore in the hope of causing a light case. The practice was risky, since some patients developed severe disease and died, but the odds were better than those of catching smallpox naturally. Lady Mary Wortley Montagu, who saw it in Istanbul, promoted it in England in the 1720s.

In 1796, the English country doctor Edward Jenner took a more cautious approach. He had heard the folk belief that dairymaids who caught cowpox, a mild disease of cattle, were protected from smallpox. Jenner tested the idea by inoculating a boy, James Phipps, with cowpox material and later exposing him to smallpox; the boy did not fall ill. By modern standards the experiment was indefensible, yet it demonstrated the principle. Jenner called the procedure vaccination, from the Latin word for cow, vacca.

The idea was extended in the nineteenth century when Louis Pasteur showed that weakened forms of a germ could protect against disease, and he developed vaccines for chicken cholera, anthrax, and, in 1885, rabies. In the twentieth century, vaccination campaigns brought remarkable results. In 1980, the World Health Organization declared smallpox eradicated, the only human disease so far to be wiped out entirely. Polio, once a cause of widespread paralysis in children, has been eliminated from most countries, though not all.

Success has created its own problem. When diseases become rare, people may no longer fear them and begin to focus on the small risks of the vaccine. Health officials therefore argue that the best evidence for vaccination is the very absence of the diseases it prevents.

The word 'it' in 'never caught it again' in paragraph 1 refers to