To carry out a successful infection, the pathogen must fulfill several conditions concerning infection titer, control of the immune response, and, most importantly, paramount infection. The environments in which the pathogen for human infections develops and grows are diverse and biologically dissimilar. For instance, the reservoirs for a patient’s disease, such as HIV, rabies, or COVID-19, can be three different reservoirs, respectively: another human, an animal, and non-living matter.
HIV, being an active suppressor of the patient’s immune system, has various transmission routes to the host, but human-to-human transmission during unprotected sexual contact is central. In the previous host, the virus is intensively duplicated and affects T cells so that when it enters the new host, HIV is already in its active phase. On the other hand, an animal, such as a dog or wild raccoon, can be a reservoir for the rabies virus’s growth and development. During a bite from such an animal, the pathogen enters the blood together with the saliva and then undergoes an incubation period. Consequently, in the previous host body, the growth and development of this virus take place, with the animal itself also being inhibited. On the contrary, the non-living environment for preserving the pathogen can be the environment: work surfaces, door handles, restaurant menus. These objects contain some inactivated forms of viruses, such as COVID-19, which, under coincidence, are still capable of causing infection if ingested through mucous membranes. It follows that although viral growth and development on such surfaces are not possible (for bacteria, this is optional), it is still acceptable to store and keep them there.