Aerosol spread or transmission is one of the ways bacteria and viruses spread from one host to a new one. It is by no means a fully effective route and the success of a particular bacterium or virus particle is dependent on many variables including environmental and host factors in addition to the attributes of the virus or bacteria
Looking at weather conditions first, it is fairly obvious that an increase in wind speed will increase the distance that the virus or bacterium travels before it lands, increasing the chance of it reaching a host. In this way some viruses can travel many miles.
If the humidity increases then bacteria or viruses will settle on the ground faster, reducing their chances of infecting a new host.
Ultra Violet light is dangerous to many viruses and so a sunny day will decrease the likelihood of any viruses transmitted still being viable when they reach the host.
Physical factors can also affect aerosol transmission: the presence of trees or buildings can cause turbulence, increasing the deposition rate of viruses and bacteria.
If there is a high density of animals then not only are there increased numbers of new hosts for the virus or bacterium to infect but there are also likely to be a larger number of viruses or bacteria released into the space.
The host's susceptibility to infection will affect the success rate of disease transmission, as will the host's tidal volume (the volume of air the host breathes in with every breath).
The number of infective particles shed by the host will obviously also affect success, as will the method in which they are shed, for example coughing or sneezing will send the particles further than merely breathing them out.
If the virus or bacterium is more resistant to dessication (drying out) or to UV light then it is more likely still to be viable when it reaches its new host. Some bacteria form spores which are particularly resistant to dessication and can survive for a long time in the environment.
The lighter the infective particle is the longer it will travel in the air before deposition, further increasing its chances of reaching a susceptible host.
Many factors affect the aerosol spread of bacteria and viruses. The best candidate for aerosol spread would be a lightweight particle that is resistant to dessication and UV light. It would be shed in large numbers by a host that has a large tidal volume that tends to live in groups with others of the same species. It would survive best on a dull day with humidity levels balanced to prevent dessication and deposition and the wind speed would be relatively high.