Mepron - what else does it treat?

Short answer: It hits bartonella, mold/fungus, and some viruses.

Longer answer: It's officially approved by the FDA to treat pneumocystis pneumonia, which is caused by a type of fungus.

From a biological point of view, it inhibits the cytochrome bc1 complex, which is present in bartonella and babesia, but not borrelia. Borrelia (Lyme) relies solely on glycolysis for ATP production, and atovaquone does not affect this.

Blocking the cytochrome bc1 inhibits energy production (ATP) but this alone usually isn't enough to kill the bacteria or parasite outright, so it's generally necessary to combine it with another antibiotic. This is why atovaquone is used in a combination pill with proguanil (malarone).

The problem with proguanil is that it is a very strong anti-folate and this can be difficult to tolerate. I felt really bad after 2 weeks of malarone, and a blood test found I had low folate. So I switched to mepron, and took it with minocycline and azithromycin.

How atovaquone inhibits viruses is less clear, but it's probably unrelated to its action on the cytochrome bc1 complex. Whatever the mechanism is, it's a quite effective treatment for coronaviruses. My experience using it to treat other common viruses (cold, flu, rsv, etc) is mixed. Sometimes it works, and sometimes it doesn't.

Mepron/zithromax caused a significant bartonella herx for me. This is likely due to bartonella's antibiotic resistance mechanism being largely dependent on efflux pumps (see https://pubmed.ncbi.nlm.nih.gov/27667026/). Since atovaquone inhibits energy production in bartonella, this removes the power source for the efflux pumps, so bartonella can no longer eject the azithromycin.

The synergistic effect in babesia is a little less direct. Babesia contains an apicoplast (endosymbiont prokaryote) which is susceptible to antibiotics. If you can kill the apicoplast with antibiotics, the babesia parasite will eventually die. The apicoplast is inside the larger parasite, but you've got to get enough antibiotic in there to affect it, and it seems that hitting babesia with atovaquone can help with this.