5. STDs:
ANSWER: C. Are very frequently asymptomatic (cause no signs or symptoms at all).
Many STDs are spread without people ever knowing they had an infection, because they are silent. Chlamydia, for example, is silent 50% of the time in men, and up to 75% of the time in women, but can go on to cause scarring and infertility in a significant percentage of women. Similarly, gonorrhea is silent in 20-40% of women and 10% of men.