The virus I work on African swine fever virus (ASFV) makes pigs very sick, it makes them bleed inside, this makes their ears red and they have blood in their poo. It is really horrible for the pigs.
It does this because inside the pig it makes the cells (the pigs own cells!) produce things which have these effects. So things that the pig normally uses to control the things that go on inside it all get out of sink and because of the virus it releases the wrong things which make it’s blood vessels leaky and that makes it bleed.
I hope this helps, please comment if you want me to explain any more!