This may be true in the case of damping-off. More often, however, much has occurred before symptoms are seen. Most pathogens cannot move on their own, but must be carried to the host plant. This is done by rain, wind, insects, birds and people. Splashing rain carries spores of apple scab fungus from infected apple leaves to uninfected leaves.
Wind blows fungal spores from plant to plant.
The spotted cucumber beetle transmits bacterial wilt of cucumbers when feeding. Working in the garden when plants are wet is a common way to spread disease. Smokers can transmit tobacco mosaic virus from a cigarette to tomato plants. Seeds or cuttings from infected plants will also transmit disease. Certified seed guarantees that at the time of sale the seeds are free of all diseases.