There are several methods by which a disease-causing organism can be transferred from the reservoir to the host organism.
Direct contact : Disease - causing organism may be transferred immediatly from reservoir or carrier to a healthy person by direct physical contact. This type of transfer is seen in diseases where the disease - causing organism cannot live for a longer time outside the body of human host. Diseases like skin and eye infections are transferred by direct physical contact.
Droplet infection : Droplets of saliva are sprayed out when we cough, sneeze or spit. These salivary droplets from sick person contain the disease-causing organisms. These salivary droplets act as a source of infection for others. Diseases like mumps, influenza, measles, chicken pox, common cold, whooping cough and tuberculosis are transmitted in this manner.
Vehicle borne transmission : In this mode of transmission, the disease-causing organism is transmitted through another living animal which is called as VECTOR. In several cases, the vectors are insects and rats or mice. Diseases such as plague, malaria are transmitted in this manner. Carriers play a very important role in the transmission of a disease. The carrier appears normal but is infected with a disease- causing organism.
Airborne transmission : Air acts as a carrier or vehicle for transferring some of the disease causing organisms. These may be released when a sick person coughs, spits, sneezes. Deseases such as tuberculosis, influenza, chickenpox, measles etc., are transmitted in this manner.
Unhygienic habits : Eating food without washing hands, eating food or drinking water from unhygienic sources also causes the transfer of disease-causing organisms.