Pathogens spread from one host to another in several ways. Direct contact includes touching, blood and sexual contact. Droplets are tiny drops that an infected person sends into the air when they cough. Someone who sneezes sends out droplets too, and others breathe them in. Pathogens can also be carried in contaminated water and food: cholera spreads in dirty water, and Salmonella spreads in contaminated food. A vector is an organism that carries a pathogen from one host to another. For example, the female Anopheles mosquito carries the protist that causes malaria.
Plants have similar routes. Fungal spores are carried by wind and rain water. Insects such as aphids act as vectors for plant viruses, and diseases can pass when infected plants touch healthy ones, through soil, or on tools and hands.
Scientists estimate how far a disease has spread by counting infected cases in a small sample. For example, if 5 of 50 plants in a sample are infected, that is 10 per cent, so a field of 1000 plants has about 100 infected plants. Larger samples give a more reliable estimate. The number of pathogens in a host also matters, because a larger number makes infection more likely and the disease more serious.