Unfortunately I discovered the answer to this last year. I was coming down the stairs at home, and crawling up the wall was a spider. Now, I am careful not to harm any of our eight-legged friends, though would prefer not to share our living space with them if at all possible. So I did what I usually do, and picked it up in my hand and headed for the front door to release it back into the wild.
Almost immediately I felt a sharp pain in the palm of my hand. The little bugger had bitten me! I quickly threw the spider away, whilst leaping around the hall-way howling and clutching my hand in agony. My Beloved managed to catch the spider with thick kitchen towel and threw it into the garden. I spent the rest of the evening running my hand under the cold-water tap, hoping the pain would ease.
The pain was about equal to a wasp sting, but it went on for a lot longer, and left a red mark in the palm of my hand for weeks afterwards. I couldn't sleep that night, as the pain extended up my arm and throbbed unbearably. The next day I rang in to work to say that I couldn't go in, much to the amusement of my colleagues who taunted me for months afterwards enquiring if I had developed spidy powers yet like Peter Parker.
As I say, that was last year, but this evening there was a familiar-looking spider in almost the same spot on the staircase wall as before. This time I got a photo of it before getting a cup over it and putting it out into the night. Now I'm not saying it is the same spider come back, but maybe it was one of its off-spring looking for revenge.
Here is a photo of it:
It is perhaps only about three centimetres long from foot-tip to foot-tip, and I think it is what in Germany is called an Ammen-Dornfinger, or more accurately cheiracanthium punctorium. From the Greek cheiro = 'hand', and akantha = 'thorn' you (sort of) get the German name Dornfinger. The thorn isn't anything to do with the spider's mandibles, but the male reproductive organs. Ouch. 'Ammen' is German for a nurse/nanny, and refers to the way the female spiders protect their offspring in a thick, swaddling web as well as her fierce protection of her young. The English name for the spider is a 'yellow sac' spider, which is in allusion to their yellow eggs.
These spiders are more at home around the Mediterranean, but with climate change they have extended their range up into Germany and Austria, causing periodic hysterical scare stories in the newspapers (e.g. 'Austria Gripped by Fear of Spider').
The good news is, if you are bitten by Germany's 'tödlichste Spinne' you would be highly unlikely and very unlucky to die of it. The bad news is, it's bite can hurt like Hell, and more and more are marching towards Berlin and Brandenburg each year as the weather gets warmer!