Hi Dotti,
We are lucky in Australia that we have different Salmonella strains in our chicken population.
Vertical transmission, is exactly what it sounds like - the salmonella is in the chook's system and is transmitted (vertically) down to the egg and ends up inside it the egg because it has colonised areas of the chickens where the eggs are formed.
The reason these salmonellas do that is because they are better able to colonise the chicken's system than other organisms that exist.
However, not every Salmonella is able to transmit vertically via the egg.
The salmonellas we have in the Australian chook population are usually not able to do this. A common salmonella in Aussie chooks is Salmonella Sofia. (Nice name!!!

)
These salmonella can still cause diseases, but they infect humans via their contact with faecal matter - not via the egg. (horizontal transmission!!)
The really cool thing about our salmonellas is that they are able to inhibit other strains of salmonella from getting a foothold. Basically, the chickens in the parent's flock already carry the "not vertically transmissible" salmonellas and colonise the young as soon as they make contact. This means the "eco-system" within the young is set up with the organisms that will be their gut bacteria scene for the rest of their lives (unless they get antibiotics). Because salmonella other than the "baddies" are already there, it would take a serious event to get a different one in.
Hence, it is less frequently that we see the vertical transmission thingy here. The use of probiotics is frequently promoted as a way of introducing protective bacteria to the gut and while it works, as long as a contaminated chicken does not come in contact with littlies before they get their gut flora safely organised, it SHOULD still be OK with respect to salmonella. (My opinion, don't quote me...)
As long as we don't let the nasty salmonellas in from other countries, this situation is likely to continue as it has for the last 50 years. AQIS etc are really strict about this, as anyone that has ever tried to import chickens from OS will attest.
I hope that helps.
Ingrid