How does a virus cause disease?
A It disrupts homeostasis and equilibrium in the body.
B It produces toxic substances that harm the body.
C It rapidly undergoes mitotic cell division that quickly smothers nearby cells.
D It form endospores in the body.

It produces toxic substances that harm the body.

Explanation:

Virus cause disease when it replicate rapidly to destroy the cells of the body by the production of toxic substances. It quickly replicate and destroy the cells of immune system to get hold on the body.

(B) The effect of most density-dependent factors on population growth is greater when population density is high.

Explanation:

A limiting factor is the one, which can limit the population of the species in an environment.

The density-dependent limiting factor is the one that exerts an effect on the population of the species depending upon its size or density.

Examples of the density-dependent limiting factor include predation, migration, disease, availability of the food, and others.

For example, if when the population of the species are densely populated, they can transmit the internal and external parasites to the host organisms through skin contact and by bodily secretions. The parasites will thrive in the densely packed host population. A reduction in the host population will reduce the incidences of infection of the host by the parasite. This will make the transmission of the parasite difficult.