By the 1970s, the acceleration of airplane travel led to fears that an epidemic would leapfrog the globe much faster than the pandemics of the Middle Ages, fears that were confirmed beginning in the 1970s by the worldwide spread of: a. the Zika virus.
b. the HIV infection
c. severe acute respiratory syndrome (SARS)
d. the Asian flu virus
e. Ebola

b. the HIV infection.

Explanation:

Around the 1970s, an increase in the use of airplane travel led to fears that an epidemic would leapfrog the globe much faster than the pandemics such as tuberculosis, leprosy, smallpox, trachoma, anthrax, scabies, etc., that were evident during the Middle Ages, fears that were later confirmed by the worldwide spread of the HIV infection, beginning in the 1970s in the United States of America.

STD is an acronym for sexually transmitted disease and it can be defined as diseases that are easily transmissible or contractable from another person through sexual intercourse. Thus, STD spread from an infected person to an uninfected person while engaging in unprotected sexual intercourse. Some examples of STDs are gonorrhea, chlamydia, syphilis, HIV, etc.

HIV is an acronym for human immunodeficiency virus and it refers to a type of disease that destabilizes or destroy the immune system of a person, thus, making it impossible for antigens to effectively fight pathogens.

Generally, contracting STDs has a detrimental effect to a patient because it causes an opening (break) or sore in the body of the carrier (patient) and as such making them vulnerable to diseases that are spread through bodily fluids e.g human immunodeficiency virus (HIV), Hepatitis, staphylococcus, AIDS, etc.

We will assume that i / 1 is constant and i express i = 1 / n while n = 1,2,3,4,5,6,7 ..