Familiars can look however the creator wants. In many cases this means looking very human, if their creator wants them to pass unnoticed through society.

And, um... well, they would fade in and out. As in, um, they'd become incorporeal and lose whatever they're holding and such, um... as for sick, basically flu-like symptoms with no visible cause. Inability to keep anything they eat down, fever, nausea, lightheadedness, that kind of thing. Not all the time, but it would happen a lot so, um, yeah.