Obligatory: clinical scientist here, not a real historian. Just an enthusiastic amateur with an excuse to excitedly ramble about one of my favourite topics in the world.

So, just to support and expand upon the succinct-and-sweet answer of Kl00k606, the medical field has advanced incomprehensibly far in the postwar period. Infectious disease has (at least compared to the entirety of human history before) become a relative footnote - in particular due to mass vaccination programmes and the absolute explosion in antibiotic discoveries during the 1950s-70s.
^(At the risk of bending the 20-year rule, the fact that this boom has largely fizzled out represents one of the more pressing threats to healthcare today, as the spread of antibiotic resistance is outstripping our ability to counteract it)

It's difficult to really hammer home how radically different public health has become since the 20th century (particularly the mid-20th-c.) compared to... well, the entirety of human existence before that.

Many of the most basic surgical technologies now universally taken for granted only proliferated during the first World War. Relevant to your question would be the large-scale use of safe, cheap, and effective antiseptic. The unprecedented flood of battlefield casualties led to experimentation with a large number of antiseptic compounds; arguably the greatest breakthrough (at least on the Western Front?) was the discovery of the Carrel-Dakin Method of asepsis. This revolved around the use of heavily diluted sodium hypochlorite - a technical term for 'watered-down bleach'.
This was 1916.

Antibiotic compounds have been recognised for millennia - famously, a herbal salve written in the 10th-century Bald's Leechbook succeeded in 2015 at killing MRSA - but almost by definition pre-scientific medicine operated more on trial-and-error than actual understanding.

Penicillin is generally accepted to be history's first true antibiotic as we'd recognise the term today - safe, effective, consistent, based on understood principles, and mass-producible. Although discovered as a compound in 1928, it was only successfully purified and industrially produced in 1942. It was seen at the time as something of a miracle, capable of snatching wounded men back from the jaws of near-certain death by sepsis, but during the war it was of course exclusively supplied to the military - a civilian who had an infection successfully take root in their body wouldn't get their turn until after the war.

The takeaway here is that it's not just leprosy - nearly all infectious diseases used to hit harder than they do today, simply because treatment options were somewhere between 'laughably inadequate' and 'totally nonexistent'.
You could very easily ask the same question about bubonic plague, which entered legend by exterminating a minimum of 1/3 of the entire European and west Asian population in less than a decade, and which today has a case mortality under 10%.

There are, for example, no less than six recorded epidemics of measles alone... just in the American Colonies... over a span of 75 years. Today the MMR vaccine is nearly universal, and the entire United States - with a population well over 300 million - reports less than 200 measles cases in an average year.

Although there were always class differences in factors such as nutrition and sanitation, disease was a fact of life (and death) that cut across all social strata. Yersinia pestis burned Roman aristocrats at the height of their power, and buried medieval royalty; vibrio cholerae struck every continent in turn and carried away millions in its passage; variola cut down an estimated half billion people in the last 200 years alone, earning itself a place in history as the only human pathogen to be systematically eradicated in return.

Cholera became manageable in 1880, and the vaccine for rabies was developed in 1885. Tuberculosis became preventable in 1921. The modern tetanus shot dates back to 1924. Polio started to get off our backs starting in 1952, and the big three of MMR backed off from harassing our kids in 1970. And we got our final revenge on smallpox in 1977. You'll note, none of these are more than 150 years old.

And to bang the hammer one final time... today, we are accustomed to our children surviving to adulthood. A human infant born in 2015 had a ~24/25 chance of surviving to their 5th birthday, and this average includes the most desperately poverty-stricken nations on earth.

According to research compiled by the Human Mortality Database and UNIGME, that global figure was closer to 4/5 in 1960. An average human born in 1920, just after the Great War, had an approximately 2/3 chance of ever seeing 1925. Continue to 1860 and the survival rate falls to just over half, where it remains as you move further back through time.

How things have changed.

Something to think about next time you head to the doctor's office.