Occasionally a new resistant strain of pathogen appears. The new strain may spread rapidly. Explain why

Antibiotics are not effective for new strains and so new resistant pathogens survive and reproduce, which leads to the increasing population of resistant pathogens and spread rapidly.

Bacteria can evolve and adapt themselves quickly as they are able to reproduce at a fast rate. Mutations of bacteria produce new strains that are resistant to certain antibiotics, such as penicillin, and cannot be destroyed by the antibiotic so they spread more rapidly.

i think it is dna, i'm not totally sure but i think it might be.

the menstrual cycle – higher tier. negative feedback mechanisms affect the production of hormones in the menstrual cycle. oestrogen also stimulates the release of lh (luteinising hormone) from the pituitary gland, which in turn controls the production of progesterone.