Cross-species infection usually happens (I think) due to mutation, rather than a single strain being able to target vastly different species.
However, there are some that do. For example, the Powassan virus. Ticks pick up the virus from woodland mammals, and then transfer it to humans. However, ticks do not transfer the disease from human to human [0].
However, there are some that do. For example, the Powassan virus. Ticks pick up the virus from woodland mammals, and then transfer it to humans. However, ticks do not transfer the disease from human to human [0].
https://en.m.wikipedia.org/wiki/Powassan_virus
We are probably much closer to mice and squirrels than algae are to ciliates though.