The main cause of mental health issues is toxoplasmosis, rather than pathogens, because toxoplasmosis is an actual explanation for a mental health condition. It is unlikely that a person gets a mental condition from breathing too much pollution, or we would have all contracted the exact same mental illness. But toxoplasmosis is a parasite. That explains it.
There's evidence that toxoplasmosis could be involved in some mental health issues some of the time. But there are many factors that could contribute.
Even neurological findings don't tell us much about the origins, since the brain can respond to many different factors, including our experiences with other people. Since social interaction involves neurons firing social interaction can alter the brain for better or for worse.
Even a genetic correlation doesn't tell us much, since with rare exceptions such as Huntington's any genes only lead to a higher risk, not a certitude. That means there are yet more factors. Some people naively suggest "curing" these genes but we don't know much about them yet, some could even lead to better outcomes under the right circumstances and then it's probably better to address the circumstances rather than fool around with the genetic code.
I'd say pathogens in general cause more mental health issues in patients. A pathogen can be just about anything that invades a human body and causes problems in the body and brain. A pathogen can even be considered evil thoughts that cause neurons to fire in parts of the brain that cause emotional upset.
Toxoplasmosis is simply an infection caused by a parasite and pathogens are a little more varied. Pathogens can include viruses, bacteria, or micro organisms that go on to cause a disease. I honeslty haven't heard of any evidence that either of these things have been found to cause any mental illnesses, much less all of them. The medical field is actually perplexed by most mental illnesses because their source is often very unclear and there is no medical test to run, to confirm them.
Pathogens are more of a cause of mental health issues than is Toxoplasmosis. Although some research can be cited by proponents on both sides of the argument, the more convincing of the empirical research cited by scientists studying the issue closely suggest that pathogens in the brain are the main cause.