Many diseases, including COVID$-19,$ spread because the virus is carried by tiny particles $($diameter ~ $10$ m$)$ suspended in air, emitted when people sneeze, cough, exhale, and talk. A recent study found that these suspended particles have an effective diffusion coefficient of $1\mathrm{.}610-3$ m$2/$s$.$ We have often been advised to stay $6$ feet $(1\mathrm{.}83$ m$)$ away from other people in order to reduce the likelihood of contracting COVID$-19.$