Final answer:
Pathogens are infectious agents that cause disease, but not all microbes on or in humans are pathogenic, and not all pathogens are parasitic. Statement 'a' about pathogens causing disease is true, while 'b' and 'c' are false.
Step-by-step explanation:
When considering which statements about pathogens are true, we need to understand what pathogens are and how they function:
Pathogens are infectious agents that cause disease.
Not all microbes living on or in humans are pathogenic; many are harmless or even beneficial.
While many pathogens are parasitic, not all of them are. Some pathogens cause disease by producing toxins or by directly damaging cells.
Infectious diseases can be caused by various types of pathogens, including bacteria, viruses, fungi, and protozoa. Some pathogens are easily treatable with medications, like those caused by bacteria which can be cured with antibiotics, while others may not be as easily treatable. Furthermore, the methods of transmission of pathogens can vary; they can be airborne, transmitted through direct contact, fecal-oral routes, or via vectors.
Based on this information, the true statements from the options provided are:
Pathogens are infectious agents that cause disease (True)
All pathogens are parasitic (False - not all pathogens are parasitic)
Therefore, statement 'a' is true, while 'b' and 'c' are false.