[FormerLurker]

Oh, I know American sanitation was so bad in the early 50s that polio was running rampant and then in just a couple of years, we (sorry I can't resist) cleaned up our act.

From Polio Transmission

Transmission

- How is poliomyelitis transmitted?

Humans are the only known reservoir of poliovirus, which is transmitted most frequently by persons with inapparent infections. There is no asymptomatic carrier state except in immune deficient persons.

Person-to-person spread of poliovirus via the fecal-oral route is the most important route of transmission, although the oral-oral route may account for some cases.

Poliovirus infection typically peaks in the summer months in temperate climates. There is no seasonal pattern in tropical climates.

Poliovirus is highly infectious, with seroconversion rates in susceptible household contacts of children nearly 100% and over 90% in susceptible household contacts of adults. Cases are most infectious from 7 to 10 days before and after the onset of symptoms, but poliovirus may be present in the stool from 3 to 6 weeks.

Source :  CDC's document about Poliomyelitis

 

The "fecal-oral route" described above could very well have come into play due to the poor hygene of migrant workers harvesting and handling produce that ended up at our supermarkets.

With better sanitation and hygene, the transmission of the disease declined.

[Sabertooth]

With better sanitation and hygene, the transmission of the disease declined.

There are, at this very minute, more people living on planet Earth in unhygenic and unsanitary conditions, than at any other time in human History. There are more today than yesterday, and there will be more tomorrow. There were more unsanitary and unhygenic people in 1970 than 1950, 1900, 1800, 1700, 1500, 1000, 500, 100 BC, 1000 BC or 1400 BC.

Why have there been no naturally occuring smallpox cases in the last 24 years anywhere in the First, Second, or Third Worlds?