Spread of communicable diseases

Transmission involves both a route and a new host. A pathogen may travel through air, water, direct contact or another organism. Reaching a new host does not always mean that infection follows, but the more pathogens that enter a body, the more likely infection becomes. Once inside, some pathogens multiply and can later spread onwards.

Routes through air, contact and water

When an infected person coughs, sneezes or talks, droplets containing pathogens can enter the air. Another person may breathe them in. Influenza and tuberculosis can spread in this way. Air can also carry fungal spores between plants; if spores land on a suitable plant, they may start an infection.

Direct contact can transfer pathogens between people, animals or plants. For example, pathogens can pass through sexual contact or enter through cuts and needle punctures. In a field, infected plant material can transfer a pathogen to another plant. A vector carries a pathogen between hosts rather than causing the disease itself; animals can act as vectors of plant or animal diseases.

Water provides another route. Splashes can carry fungal spores from an infected plant to a healthy one. In humans, drinking contaminated water or eating contaminated food can bring pathogens into the digestive system. Crowding and poor sewage disposal can increase opportunities for transmission, so the conditions in which organisms live affect how quickly a disease spreads.

Measuring and estimating spread

Two quantities answer different questions. The number of pathogens describes how many disease-causing particles or organisms are present in a sample or exposure. The number of infected cases counts infected hosts. A single case may involve many pathogens, so these numbers must not be treated as interchangeable.

To examine spread, count new infected cases over equal time intervals and display them in a table or on a graph. Rising numbers suggest that transmission is occurring, but a graph alone does not identify the route or prove its cause. Check the axis labels and whether the figures show new cases in each interval or a running total of all cases so far.

Data for Illustrative count of new cases

It may be impossible to test every individual in a large population. Instead, test a sample and use its infected proportion to estimate cases in the whole population. For example, if 20 people are infected in a sample of 200 people, the sample proportion is $20/200=0.10$, or 10%. Applied to a population of 4,000 people, this gives an estimate of $0.10\times4,000=400$ infected people. The estimate is more trustworthy when the sample represents the wider population; a sample drawn only from people already seeking treatment could exaggerate the proportion infected.