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The answer to this is:
It is invalid because it could not stand up to experimentation
The human body is a complex and thriving ecosystem. It contains about 1013 human cells and also about 1014 bacterial, fungal, and protozoan cells, which represent thousands of microbial species. These microbes, called the normal flora, are usually limited to certain areas of the body, including the skin, mouth, large intestine, and vagina. In addition, humans are always infected with viruses, most of which rarely, if ever, become symptomatic. If it is normal for us to live in such close intimacy with a wide variety of microbes, how is it that some of them are capable of causing us illness or death?
Pathogens are usually distinct from the normal flora. Our normal microbial inhabitants only cause trouble if our immune systems are weakened or if they gain access to a normally sterile part of the body (for example, when a bowel perforation enables the gut flora to enter the peritoneal cavity of the abdomen, causing peritonitis). In contrast, dedicated pathogens do not require that the host be immunocompromised or injured. They have developed highly specialized mechanisms for crossing cellular and biochemical barriers and for eliciting specific responses from the host organism that contribute to the survival and multiplication of the pathogen.
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