If we travel back in time, as far back to 429BC, Thucydides who was an Athenian historian and general, recognised smallpox reinfection did not occur to those who survived the plague.
Similarly, to vaccination for smallpox and other strains of disease, those who become infected unnaturally (via the vaccination process) do not hold complete immunity and the level of immunity decreases with time.
In 1796, the first vaccine which was created by English physician and scientist Edward Jenner for smallpox was considered in the medical world as the 'The father of immunology'.
The last naturally occurring case of smallpox (Variola minor) was diagnosed on the 26th of October, 1977.
Smallpox although more relevant in the 70's "was one of the biggest human scourges," said by HHMI international research scholar Gunasegaran Karupiah, a scientist at the John Curtin School of Medical Research at the Australian National University. Apparently 30% of those infected became deceased. As of today the active virus is only available in the Center for Disease Control and Prevention (CDC) in Atlanta and the Russian State Centre for Research on Virology and Biotechnology located in Moscow.
From 1982, the smallpox vaccine was suspended to the public, although again resumed in production mode only when Ken Alibek, a former deputy director of the Former Soviet Union's bioweapons program identified how smallpox could potentially be a bioterror threat. A limited number of people have been vaccinated since the suspension as part of a military response team to the September 11th tragedy.
Interestingly George W. Bush referred to stocking the smallpox vaccine to protect the entire country.
The vaccine was introduced well over a hundred years before the smallpox epidemic began to slow down and vaccination was compulsory in the UK, USA and most of Europe for over fifty years before the epidemic lost its strength.
Not too long after the original bill mandating the smallpox vaccine (1853), a severer bill which had inclusions of huge fines and imprisonment for vaccination refusal was implemented in 1868. It was only four years after this legal obligation, that the worst spike of smallpox occurred.
Protests occurred revolting against mandatory vaccination for a variety of reasons including new science scares, offense taken to something being derived from a cow and then injected into the human body, distrust and mistrust to the health system, the process being unnatural/ not 'Gods way' and a violation of personal liabilities. Many ethical issues that are still being continuously raised today.
Eventually these protests brought in new measures to manage smallpox including stringent quarantine rules for those infected and their families. Very soon after this, the smallpox epidemic lost its power.
Diseases have greatly reduced over time without a vaccine including bubonic plague, scarlet fever, tuberculosis and syphilis.
Considering the largest outbreaks of smallpox occurred in England in the 1870s after the vaccine was made compulsory; There is still serious debate today on whether the vaccination was the actual reason for eradication or whether the disease naturally declined from improved sanitation.