Aim of this work is to verify difference in spread attitudes of covid-19 and other coronavirus versus Smalpox and try to seek rational explain of the specific pattern of diffusion. The rapid increase of covid- 19 cases in FRANCE in second wave in fisrt day of october 2020 seem to follow not only a diffusion related direct contact and by droplet but also by airborne trasmission. A real fact is the diffenece in size of virus smalpox vs coronaviruses. Virus size is an relevant factor involved in kind of spread.