Final answer:
The term 'infective dose' refers to the number of microorganisms necessary to establish an infection, with the correct answer being the student's option (a). It is quantified in laboratory settings as the median infectious dose (ID50).
Step-by-step explanation:
The term 'infective dose' refers to the number of microorganisms necessary to establish an infection; it varies from a few to several million, depending on factors such as the virulence of the infectious agent and the host defenses. The infective dose is therefore the correct answer to the student's question. This concept is closely related to the pathogen's virulence factors and the host's immune defenses. In disease perspective, the median infectious dose (ID50) is a measure used to quantify a pathogen's virulence, which is the number of pathogen cells, virions, or amount of toxin required to cause infection in 50% of a test population.
There are several periods of a disease that include the incubation period, the prodromal period, the period of illness, the period of decline, and the period of convalescence. The incubation period is when the pathogen begins multiplying in the host but has not yet caused signs and symptoms of the disease. Various factors, such as the size of the infective dose, play a role in determining the length of the incubation period.