Which of the following is considered a major risk factor leading to poliomyelitis outbreaks?

Lack of sanitation is recognized as a major risk factor leading to poliomyelitis outbreaks because the poliovirus primarily spreads through the fecal-oral route. In areas with inadequate sanitation, such as insufficient access to clean water and proper sewage disposal, the likelihood of virus transmission increases significantly. People can be exposed to contaminated water or food, facilitating the spread of the virus among individuals, particularly in communities where hygiene practices are poor.

In contrast, airborne pollution, close contact with animals, and high vaccination rates do not contribute to the transmission of poliovirus in the same way. Airborne pollution may affect general health but does not directly spread the virus, while close contact with animals is not a transmission route for poliomyelitis. High vaccination rates, on the other hand, are crucial in preventing poliomyelitis outbreaks as they lead to herd immunity, reducing the virus's transmission opportunities within a population.