Pathogens spread in several ways. Direct contact includes touching, body fluids and sexual contact. Air: coughs and sneezes release droplets that carry viruses and bacteria. Water and food can be contaminated with pathogens, such as cholera bacteria. A vector is an organism that carries a pathogen from one host to another. A mosquito is the vector for the malaria protist.
Plants are infected in similar ways. Fungal spores are blown by the wind, water and soil carry pathogens, and plants touching each other can pass them on. Insects such as aphids are vectors that carry viruses from plant to plant.
Scientists count the number of pathogens and the number of infected cases. They often sample, because they cannot check every plant or person. If 30 of 150 sampled plants are infected, that is 20%. In a field of 5000 plants, they estimate that 1000 are infected.