.data() binding only first element

.data() binding only first element

I am building an epidemic simulation using D3's force-directed diagram.
When a transmission event occurs, I want to move a circle from the
transmitter to the newly infected individual.
PROBLEM: Only the first element is created and moved according to the
bound data.
First, I gather the coordinates:
xyCoords = getPathogen_xyCoords(newInfections);
Where xyCoords looks like the following:
{receiverX: newInfections[i].x, receiverY: newInfections[i].y,
transmitterX: newInfections[i].infectedBy.x, transmitterY:
newInfections[i].infectedBy.y}
Then I create the circles and bind them to xyCoords:
d3.select(".svg").append("circle")
.attr("class", "pathogen")
d3.selectAll(".pathogen")
.data(xyCoords)
.attr("cx", function(d) { return d.transmitterX})
.attr("cy", function(d) { return d.transmitterY})
.attr("r", 4)
.style("fill", "green")
Finally, the circle is moved with a transition:
d3.selectAll(".pathogen")
.transition()
.duration(500)
.attr("cx", function(d) { return d.receiverX} )
.attr("cy", function(d) { return d.receiverY} );