The Spanish Flu started during WWI, not after it.
True, the Spanish Flu started during WWI, but it did not become a pandemic until after the war ended and all the soldiers and refugees carried the disease home with them.
In a similar vein while AIDS had killed a few people in Africa, it was not until large numbers of soldiers loaned there by Haiti and Cuba went home that it became a wide spread disease. It is theorized that gay men partying in Haiti and Haitian immigrant truck drivers visiting prostitutes in a town at the south end of Lake Okachobie (Belle Glade?) in Florida were the main cause of the sudden outbreak in the US. Cubans did not contribute because they were not allowed to travel and the Cuban government’s immediate reaction was to quarantine the ill. Later they improved treatment both of the disease and the people.
This 2005 article discusses the idea that in Europe a gene (CCR5-delta32) seems to confer 10% protection from HIV/AIDS, and that it is connected with plagues like Black Death (Yersinia pestis).
http://www.eurekalert.org/pub_releases/2005-03/uol-bdw031005.php
And this report (2006) which casts doubt on the connection, but recognizes the value of correlating a lot of different data regarding possible correlation of body defenses.
http://qjmed.oxfordjournals.org/content/99/8/497
This more recent site (2013) lists newer study of the possible uses of the CCR5-delta32 genetic factor in treating/curing AIDS and other illnesses.
http://www.genomebiology.com/2013/14/1/201
It has been noted that there is a lot less active plague even in poorer areas than before. Several possible factors come to my mind. Better nutrition and especially of foods elevating Vitamin C in the blood no doubt help. In Europe, and perhaps elsewhere, the black rat a house and roof dweller seemed to be a major culprit. These days in Euorope and US, the Norway rate is the major species and it is a sewer and cellar rate with less contact with humans.