Plague has always traveled long distances by ship. A ship creates the perfect environment for containing, incubating and magnifying the contagion. Even so, we don’t have very many descriptions of plague ships. Gregory of Tours may provide perhaps one of the earliest description in his History of the Franks (IX:20-21).
“At this time (588 AD) it was reported that Marseilles was suffering from a severe epidemic of swelling in the groin and that this disease quickly spread to Saint-Symphorien-d’Ozon, a village near Lyon. … I want to tell you exactly how this came about. … a ship from Spain put into port with the usual kind of cargo, unfortunately also bringing with it the source of the infection. Quite a few of the townsfolk purchased objects from the cargo and in less than no time a house in which eight people lived was left completely deserted, all of the inhabitants having caught the disease. The infection did not spread through the residential quarter immediately. Some time passed and then, like a cornfield set alight, the entire town was suddenly ablaze with pestilence. … At the end of two months the plague burned itself out. The population returned to Marseilles, thinking themselves safe. The disease started again and all who had come back died. On several occasions later on Marseilles suffered from an epidemic of this sort.”
Gregory of Tours, History of the Franks, IX: 20-21. Lewis Thorpe, trans. Penguin. p. 509-511
Bubonic plague, identified by the severe epidemic with a swelling in the groin. Gregory often refers to it as inguinal pestilence. This passage is a classic description of what we should see for bubonic plague. Gregory specifies that the initial victims handled cargo from the ship; no mention of ill crew or passengers.
Mind the gaps. There was a passage of time between the first eight victims and the explosion of pestilence throughout Marseilles. This would be when the rat epizootic was occurring. It explodes throughout the city when the rats are dead and the fleas move on to humans. Contagion transmitted by cargo is the movement of fleas only. I doubt many rats left plague ships. When the Marseilles rats come to feed on the cargo, they contract the infection from fleas in the grain, other foodstuffs or textiles.
Many of the people of Marseilles must have fled, as they often do before the plague, to return when word got out two months later that the sickness was gone. The second wave of pestilence was triggered by contagion left in the town by persistent fleas, possibly still in foodstuffs. If the returning people brought new rats into the town with them by bringing in fresh supplies, that could also restart the epidemic. The more intense the initial epidemic was, the more bacteria would be scattered in the environment to infect the returning people.
Gregory of Tours died in 594, so the several epidemics in Marseilles that he refers to must have happened between 588 and 594. Marseilles was one of the busiest Frankish ports because it was one of their only ports on the Mediterranean sea. It is likely that over at least the next century, many plague ships visited the port of Marseilles.