What process involves one organism's introduction to another's environment causing a disease?

The process of one organism's introduction to another's environment leading to a disease is best described as disease transmission. This refers to the mechanisms by which infectious agents are spread from one host to another, facilitating the infection of the new host. This can occur through various routes, such as direct contact, droplet spread, airborne transmission, or vectors like insects.

Infection refers to the result of disease transmission when the pathogen successfully invades and multiplies within the host, but it does not encompass the broader concept of the transfer itself. Contamination involves the presence of infectious agents on surfaces or in materials without necessarily causing an infection. Colonization is the establishment of a microbe in a host without causing disease; it indicates a passive presence rather than an active disease transmission process. Thus, disease transmission accurately captures the concept of transferring disease-causing organisms between hosts, making it the correct choice.