Answer:
The correct answer is:
Both V. cholerae and S. typhi are pathogenic, but S. typhi is more virulent than V. cholerae. (B)
Step-by-step explanation:
The ID50 of a pathogen measures the smallest size of an infectious agent that has a 50% probable chance of causing an infection in an individual.
The higher the Infectious dose (ID), the lesser the virulence of the organism, and the lesser the infectious dose, the higher the virulence of the organism. In the example given in this question, the infectious dose of Vibrio cholerae with a 50% chance of causing an infection is 1,000,000 cells while that of salmonella typhi is 10,000 cells, meaning that the population of V. cholerae cells have to accumulate to 1 million cells before the chance of causing infection is 50%, while for S. typhi, only 10,000 cells creates this chance, hence S. typhi is 100 times more virulent than V. cholerae.
ID50 is measured by studying the extent of phagocyte subversion or the ability to kill phagocytes. Certain factors e.g., immunocompetence, genetics and route of exposure can affect the infectious dose of a pathogen, hence, ID is measured in a healthy individual. When pathogens enter the body, under favorable conditions, they multiply, the phagocytes are activated, to help combat the disease causing pathogens, but most pathogens have the ability to subvert or kill phagocytes and this largely depends on population size, so a pathogen like V. cholerae discribed above, before it multiplies to a size of 1,000,000 in order to have enough concentration to overcome the immune system, they may have been neutralized by the pagocytes, hence the low ID50, but for S. typhi, once it gets to a population of 10,000, its effect on the immune cells are felt resulting to a disease, hence its high ID50 and virulence.