Which of the following are Koch's postulates to prove a causal relationship between a microorganism and disease?
a. The microorganism must be present in every case of the disease and absent from healthy individuals.
b. The putative causative agent must be isolated and grown in pure culture.
c. The same disease must result when the cultured microorganism is used to infect a healthy host.
d. The same microorganism must be isolated again from the diseased host.
e. The microorganism must infect more than one individual of a particular species.
f. The microorganism must be isolated from humans only.