Answer:
a virus, because it requires a host cell to reproduce
Step-by-step explanation:
- a bacterium, because it is contagious
- a fungus, because it infects livestock
- a protist, because it is microscopic
- a virus, because it requires a host cell to reproduce
The correct answer would be a virus because a virus is the only microbe that cannot reproduce on its own without a host. A virus utilizes the genetic mechanism of its host to reproduce either lytically or lysogenically.
In lytic cycle, the virus utilizes the genome system of the host to produce more viruses that eventually leave or lyses the host to become independent viral particles, killing the host in the process.
In lysogenic cycle, the virus inserts its genome into that of the host and uses the host's replication machinery to replicate its genome. The virus then waits for the defense mechanism of the host to become weakened before multiplying and lysing the host.