Both Streptococcus Pyogenes and Corynebacterium Diphtheriae Are Highly Pathogenic, and the Disease

Both Streptococcus pyogenes and Corynebacterium diphtheriae are highly pathogenic, and the disease symptomology can be very severe. Why is this?

A) Infants and children are the high-risk groups, and their mortality rate is higher than for other ages.
B) Both bacteria produce potent exotoxins.
C) They induce autoimmune states in the patients.
D) There are no antibiotics useful for diseases caused by these bacteria.