Infections are classically thought to be caused by a homogeneous population of microbes (meaning a group of microbes of the same species but are all the same- think of them as clones of each other).
However, this idea is currently changing- we are now thinking that infections can be caused by a heterogeneous population of microbes (meaning a group of microbes which are all the same species but are all different).
One really important consequence of this idea is that if an infection is in fact caused by a group of different microbes is that these microbes could have different drug susceptibilities. This could create a treatment nightmare!
The image below shows a group of fungal strains isolated from a patients lungs on a single day. These strains are of the same species of fungus, but as you can see from their appearance when grown on a petri dish, they are very different. They all possess different drug susceptibilities- which means that a treatment that may kill one of them, may not kill the others. Scary stuff!
Hopefully this post highlights that infections can be caused by multiple microbes at once and how this is important clinically.