The bacteria in biofilms making people sick are species of the genus Mycobacterium. Mycobacteria are different from most waterborne pathogens in that their normal habitat is not the human body. Instead, they live in the pipes themselves and become problematic only when they, quite accidentally from the perspective of their own well-being, make their way into human lungs.
The Mycobacterium species in showerheads are typically referred to as NTM, for nontuberculous mycobacteria. This means, as you may have inferred, other mycobacterial species are tuberculous, namely the species Mycobacterium tuberculosis and its close relatives.
Mycobacterium tuberculosis appears to have long associated with humans and our extinct relatives. The dangerous form of the pathogen evolved at about the time modern humans moved out of Africa, and spread with us as we moved across the globe. Once we domesticated animals, Mycobacterium tuberculosis evolved into Mycobacterium caprae in goats and Mycobacterium bovis in cows. We gave Mycobacterium tuberculosis to mice and seals, in which it evolved yet other forms. The seal version appears to have traveled to the Americas no later than 700 C.E., where it infected Native Americans (and then evolved into yet another specialized form).
In each case, the bacteria rapidly evolved special traits enabling them to better survive and spread among individuals of each new host. Mycobacterium tuberculosis is an emblematic example of evolution’s mechanisms every bit as elegant as that offered by the differences in beak shape among the species of Darwin’s finches.
Antibiotics, first developed in the 1940s, allowed us to gain a real victory against Mycobacterium tuberculosis, but today many strains of tuberculosis bacteria are resistant to most antibiotics. These resistant strains are (predictably) spreading. All of this is to suggest that the lineage of mycobacteria is one about which it would be good to have a robust awareness.
Read: What has more germs than a toilet seat? Everything.
So far, the risk of infections due to nontuberculous mycobacteria is high only for immunocompromised people, people whose lungs have an unusual architecture, and people with cystic fibrosis. In these individuals, the pathogens can cause pneumonia-like symptoms, as well as skin and eye infections. Unfortunately, the risk of nontuberculous mycobacteria infections is increasing overall in the United States, but just how common infections are and how much more common they are becoming varies geographically.
In some regions, such as California and Florida, infections are common. In others, such as Michigan, they are rare. This difference could be due to differences in either the abundance or the presence of mycobacterial species in various regions. The species in Florida, for example, don’t seem to be the same as those in Ohio, and this might matter. Also, the mycobacterial species associated with infections tend to be the same species and strains found in showerheads, which are different from those associated with soil or other wild habitats.