ABSTRACT
Vertical transmission describes the transmission of a disease agent from animals of one generation to subsequent generations. Modes of transmission may be broadly classified as horizontal or vertical, and within horizontal as direct, indirect, or airborne. Transmission may occur directly, as from an infected person or animal, or indirectly through an intermediate plant or animal host, vector, or the inanimate environment. Infectious agents may originate from environmental sources, or part of the normal flora such as the bacterial secondary invaders responsible for pneumonia, wound infections, and abscesses. The disease agent is transmitted between hosts on soiled appendages or the proboscis, or by passage of organisms through the gastrointestinal tract. The life cycle of a disease agent may be defined as the sequence of developmental stages from infection of one host to infection of a second host. Mechanical transmission results from simple mechanical carriage of the disease agent between hosts by crawling or flying arthropods.
