  Identifying the relative importance of the different transmission routes of
the SARS-CoV-2 virus is an urgent research priority. To that end, the different
transmission routes, and their role in determining the evolution of the
Covid-19 pandemic are analyzed in this work. Probability of infection caused by
inhaling virus-laden droplets (initial, ejection diameters between $0.5-750\mu
m$) and the corresponding desiccated nuclei that mostly encapsulate the virions
post droplet evaporation, are individually calculated. At typical,
air-conditioned yet quiescent indoor space, for average viral loading, cough
droplets of initial diameter between $10-50 \mu m$ have the highest infection
probability. However, by the time they are inhaled, the diameters reduce to
about $1/6^{th}$ of their initial diameters. While the initially near unity
infection probability due to droplets rapidly decays within the first $25s$,
the small yet persistent infection probability of desiccated nuclei decays
appreciably only by $\mathcal{O} (1000s)$, assuming the virus sustains equally
well within the dried droplet nuclei as in the droplets. Combined with
molecular collision theory adapted to calculate frequency of contact between
the susceptible population and the droplet/nuclei cloud, infection rate
constants are derived ab-initio, leading to a SEIR model applicable for any
respiratory event - vector combination. Viral load, minimum infectious dose,
sensitivity of the virus half-life to the phase of its vector and dilution of
the respiratory jet/puff by the entraining air are shown to mechanistically
determine specific physical modes of transmission and variation in the basic
reproduction number $\mathcal{R}_0$, from first principle calculations.
