Final answer:
Two factors that drive the transmission rate of a virus are the contact between individuals and the viral load. The density and size of the population are wider scale factors that influence transmission risk. Increased human population and density have historically led to a higher prevalence of infectious diseases.
Step-by-step explanation:
The question asks what two things drive the transmission rate of a virus. The correct answer is d) Contact between individuals and viral load. Contact between individuals is a critical factor because the more people interact with each other, especially in close quarters, the easier it is for a virus to spread from person to person. Viral load refers to the amount of virus in a sample of an infected person's blood, which can influence how infectious an individual is to others.
On a larger scale, aspects such as the size of the population and the density of the population can influence how rapidly a virus spreads across a community or region. These factors contribute to the overall risk of transmission and are heavily influenced by environmental and social conditions.
Environmental factors, along with human behavior such as global travel, have contributed to the increase in the size and density of human populations, resulting in greater prevalence of infectious diseases. Historical evidence suggests that increased contact among individuals in larger populations has heightened the potential for the transmission and sustained presence of infectious diseases, such as measles, which requires a sizeable population for its continued existence.