Sometimes it makes people sick, and other times, prolonged exposure tranfsers immunity..
Milkmaids used to not get cow-pox (small pox) but the general public was very susceptible...
Dr Jenner figured it out
http://en.wikipedia.org/wiki/Edward_JennerSmallpox
Around this time smallpox was greatly feared, as one in three of those who contracted the disease died, and those who survived were often badly disfigured. Voltaire, a few years later, recorded that 60% of people caught smallpox, with 20% of the population dying of it. In the years following 1770 there were at least six people in England and Germany (Sevel, Jensen, Jesty 1774, Rendell, Plett 1791) who had successfully tested the possibility of using the cowpox vaccine as an immunization for smallpox in humans. <6> For example, Dorset farmer, Benjamin Jesty, had successfully induced immunity in his wife and two children with cowpox during a smallpox epidemic in 1774, but it was not until Jenner's work some twenty years later that the procedure became widely understood. Indeed it is generally believed that Jenner was unaware of Jesty's success and arrived at his conclusions independently
Jenner's Initial Theory
In fact he thought the initial source of infection was a disease of horses, called "the grease", and that this was transferred to cows by farmworkers, transformed, and then manifested as cowpox. From that point on he was correct, the complication probably arose from coincidence.
Noting the common observation that milkmaids did not generally get smallpox, Jenner theorized that the pus in the blisters which milkmaids received from cowpox (a disease similar to smallpox, but much less virulent) protected the milkmaids from smallpox. He may have had the advantage of hearing stories of Benjamin Jesty and others who deliberately arranged cowpox infection of their families, and then noticed a reduced smallpox risk in those families.
On 14 May 1796, Jenner tested his theory by inoculating James Phipps,{1788-1853} a young boy of 8 years old, with material from the cowpox blisters of the hand of Sarah Nelmes, a milkmaid who had caught cowpox from a cow called Blossom<7>, whose hide hangs on the wall of the library at St George's medical school (now in Tooting). Blossom's hide commemorates one of the school's most renowned alumni. Phipps was the 17th case described in Jenner's first paper on vaccination.
Jenner inoculated Phipps with cowpox pus in both arms on the same day. The inoculation was accomplished by scraping the pus from Nelmes' blisters onto a piece of wood then transferring this to Phipps' arms. This produced a fever and some uneasiness but no great illness. Later, he injected Phipps with variolous material, which would have been the routine attempt to produce immunity at that time. No disease followed. Jenner reported that later the boy was again challenged with variolacious material and again showed no sign of infection.
Known: that smallpox was more dangerous than variolation and cowpox less dangerous than variolation.
The hypothesis tested: That infection with cowpox would give immunity to smallpox.
The test: If variolation failed to produce an infection, Phipps was shown to be immune to smallpox.
The consequence: Immunity to smallpox could be induced much more safely.
He continued his research and reported it to the Royal Society, who did not publish the initial report. After improvement and further work, he published a report of twenty-three cases. Some of his conclusions were correct, and some erroneous—modern microbiological and microscopic methods would make this easier to repeat. The medical establishment, as cautious then as now, considered his findings for some time before accepting them. Eventually vaccination was accepted, and in 1840 the British government banned variolation- the use of the smallpox itself- and provided vaccination- using cowpox- free of charge.