A note about safety

A lot of my friends are jokingly apt to say "Safety Third!"
Here's the deal though. This is an old house. Old houses are inherently more dangerous than new houses, for a few reasons.

"Fibrous tremolite asbestos on muscovite" From Wikipedia

1) Asbestos. This mineral is dangerous if inhaled. It was often used as an insulation, particularly around old furnaces and heating pipes. It's also found in some old siding, because it's darn near impervious to weather if properly maintained. If you have it and you can, just Leave It Alone. Do not break it. Do not make it into anything inhale-able. It's not going to hurt you by looking at it. It's probably not wise to handle it, because by handling it, you're probably creating dust. Don't do that. Unless it needs to be removed, leave it alone. And if it needs to be removed, you're probably best leaving it to professionals. I didn't have any of this, and I'm very, very thankful.

This is what mine sort of looked like. Photo fromEnvironix.

2) Mold. Though you can find this in new homes too, old homes just have had a longer leadtime to get a head start on moisture where you don't want it, and then not-so-suddenly, you have a fungus among us. (Well, fungus among you. I got mine mitigated.) This stuff isn't fun, and can cause a host of health issues. I happen to be allergic to mold. Little spots you can deal with yourself, larger jobs (like up in my attic) might need the help of a pro. Read up on it. Attic mold is not so simple to deal with as, "hose it down with bleach and hope". The little roots of the mold get into the wood to hide there and come back when the time and environment is right.

Knob and Tube. From Wikipedia.

3) Knob and Tube Wiring (K&T). Like asbestos, is not dangerous if left alone in it's original environment. The problem is us modern folks like a nifty thing called insulation. Unless we're gutting the place, older homes usually get retrofitted with blown-in insulation. That's when K&T becomes a huge issue.
K&T is bare metal wire stretched between ceramic insulators in the hollow cavities in your wall that carries electricity from the main box to all the outlets and light fixtures. The wire gets through studs and joists with the help of tubes inserted into holes, shielding the wood from the wire. When you blow in insulation on top of the bare wire, and a current is applied, that's where things go south in a hurry and you can burn your house down.
A lot of times older homes can have a combination of systems as things got added on as times changed. I never had K&T in this house, (thanks for the inspection Wayne!) but there is a mix of fabric covered wire, metal shielded wire, and modern plastic coated wire. Most insurance companies with flip a nutty if they find out you have K&T. I'd have to think long and hard about getting into a property with K&T, because you're looking at having to rewire the place up to code if you want insurance.
There's newer low-voltage systems now that can approximate the look of K&T without the danger since LEDs require so little juice. I have no first-hand experience with these systems other than to say I saw it once demonstrated at a conference. I thought it was neat but not particularly practical.

I make sure to follow these directions when working with the dry product too.
Photo from Wikipedia.

4) Lead. This one have been heavy on my mind because I've been doing a lot of stripping/sanding.
If your house was built before 1978, there's a very high change your house contains lead paint. Again, much like asbestos and K&T, if you leave it alone, it won't hurt you by existing. The big problem comes with absorbing it into your body. With little kids, this looks like chewing on windowsills.

(Something I never understood why anyone would do, and yet my mother admonished me to Never Ever Do It. One day I rested my top teeth on the window sill of my bedroom, just because. The toothmarks may still be there, faintly. I don't really think it counts as chewing. And besides, I'm pretty sure the house was built in '78, so I was doubly safe. Thanks Mom. :) )

In my world today, what that looks like is dust. Lots and lots of dust, which I can breathe or get on me and absorb. Now people might say "Whatever. We've lived around lead our whole lives and we're fine!" And that's true. But it's different living around it and living immersed in it.
I met a guy who was an old window restoration geezer. He'd been working on windows for probably 50 years. He's also the only guy I've ever met who had to have his blood pumped out of his body, filtered through a glorified coffee filter, and reinstalled back in due to acute lead poisoning. Someone had screwed up the air handling at a site he was working on, and he wound up inhaling a great deal of it.
Trust me: You just don't want to be that guy.
And so, Personal Protection Equipment to the rescue!
I have on my way to me certain soaps that claim to de-lead you. I don't know how much is science, but a lot of the window and stained glass people use it, so I will too. I used to just wash up really thoroughly, but there's a peace of mind with magic lead soap.
I also employ my favorite respirator with filters rated to p100 (lead).

Safety first!

I just ordered new filters for that too.

There's other things about old houses that can be unsafe - stair run/rises not up to current code, unstable foundations, forgotten wells that have the gradually filled in but not entirely, electice that isn't labeled right, etc, but those four make the top of my list.

**DISCLAIMER - I'M NOT A PROFESSIONAL,
AND ALL OPINIONS EXPRESSED IN THIS POST ARE EXACTLY THAT.
DON'T GO BUILDING THINGS OUT OF ASBESTOS AND PAINTING THEM
IN LEAD JUST FOR KICKS, MMMKAY?**