If the harmless microbes are taking up all the growing space, then the bad bugs can't move in. This is the risk of taking broad spectrum antibiotics unnecessarily. Parasitism - a relationship between host and microbe where one benefits and the other is harmed. Viruses, all eventually kill the host cell. Most are not parasitic, but there are about 300 species of human pathogens that are parasitic in their relationship with us.
Pathogens are microbes that cause disease.
That your immune system, your natural protective mechanisms to stop infection, isn't working properly. We are born that way. Humans are not born with complete immune function. Children less than 10, particularly babies, are somewhat immune suppressed. The elderly too.