The concept of communication via breath is introduced under the molecular com munication system, where data can be exchanged through inhalation and exhalation. Those data are carried by volatile organic compounds (VOCs) or pathogens and transferred through an aerosol channel. In this thesis, we propose a molecular com munication model for an instantaneous source in a bounded indoor environment. The walls of this environment could be reflectors and/or absorbers by adjusting the value of deposition velocity. We assume a puff source in a given location and study the per formance of a point source since it is the basic element that can be used to derive the concentration of breath, cough, and sneezing, where the concentration of continuous source can be found by integrating a point source over space and time domains. Also, we show some numerical results to visualize the performance of these mathematical models and evaluate them. As a case study, we consider a real-life scenario of detecting a virus from an exhaled breath of a person standing in an indoor bounded room with reflective and absorptive walls. We derive the spatial-temporal concentration of an exhaled virus at the molecules source and the receiver in the room. Finally, we study the probability of misdetection using a suitable bio-sensor.