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Thank you for your query.
Antibiotics are derived from certain fungi, bacteria, and other organisms that can help destroy or prevent the growth of other micro-organisms (disease causing). A synthesis from fungi, bacteria or other organism takes place, through which we derive/take that part of the organism which would help in destroying or preventing the growth of other micro-organisms that cause diseases.
Once this part is derived, we combine it with a few other components and make medicine out of it (liquid, powder or solid form).
Resistance varies from person to person and also depends on the type of pathogen you are infected by. Causes for drug resistance
are many, most commonly evolution; due to which pathogens develop and change, resulting in becoming resistant to an otherwise effectively used antibiotic. Other reasons can be genes (individual), excessive antibiotic use in individuals, etc.
I hope I have succeeded in providing the information you were looking for. Please feel free to write back to me for any further clarifications. I would gladly help you.