1868: Texas Fever
A strange fact was noticed by cattlemen in the United States starting around 1868... and by 1885, it threatened to destroy a large part of the economy in the state of Texas. The odd fact was that when longhorn cattle were being transported from Texas to market in other states, non-longhorn cattle that either mixed with them or that simply used a pasture that longhorn cattle had stopped at previously would often become very ill and die. This happened far too often to be mere chance. The disease had many names, but the most infamous was "Texas Fever", which directly blamed the source of the longhorn cattle for the problem.
The fact that only the mere presence of the longhorn cattle was a threat to other cattle led to the state of Kansas, a neighboring state to Texas and gateway to many northern cattle markets, to outlaw the passage of longhorn cattle across their borders in 1885, greatly impacting the cattle market in Texas. Research began immediately to try to identify the cause and find a cure. It was clear that the longhorn cattle were somehow immune to a disease they were transmitting to other cattle; and that this disease could exist in a field after they left... so first suspicians were that their manure was somehow transmittng the illness.
But it wasn't that simple. The culprit was eventually tracked down to two related protozoans now named Babesia bigemina and Babesia bovis which live as parasites in Texas cattle. As calves, longhorn cattle are partially immune to these protozoa, so their first encounter with it isn't fatal. As they grow older, their immune systems just learn to live with the disease, leaving the cattle reasonably healthy but also carrying the protozoans in their blood. The calves got the disease from ticks... and ticks, in turn, got the disease from the cattle. When a tick sucked a bovine's blood, it picked up the protozoans; and when the tick laid eggs, using the blood to nurish them, the young ticks that were born up to twelve weeks later would also be carrying the protozoans, ready to transmit to new cattle. This was what made fields occupied by longhorns dangerous for weeks afterwards to cattle that had never encountered the protozoans before.
Once the relation of ticks to the transmission of the disease was understood, cattlemen started to 'dip' their cattle, cover them with chemicals that killed existing ticks and made the cattle non-attractive to new ticks. This dipping, combined with regulations separating southern and northern cattle in stockyards and transport centers, evenually led to the eradication of Texas Fever, and the end of the federally required dips in 1943.