Limiting factors are factors that can change growth rate of the populations. Limiting factors are the availability of food, water, shelter, disease or relationships between organisms like competition for resources or predation. So, for example, if one animal's population decreases (due to disease), the population of animals that eats that animal might also decrease (due to decreased food).
Infectious agents might be transmitted through the direct contact (person-to person or droplet spread) or indirectly through the airborne transmission, contaminated objects, insect bites, water and food, animal-to-person contact..