This is long and full of stuff so feel free to skip if you don’t care how stuff works:
An ash item would be fine, but really if you have wood + heat source then you have ashes. So it’s just an unnecessary extra step to add ashes as a crafting ingredient.
I have never heard of soap being made from bleach or ammonia. ‘Soap’ is made by mixing animal or vegetable fats with an alkali salt, such as lye (sodium hydroxide, or potassium hydroxide which is easily liberated from ash). You can even make a ‘soap’ from other alkali salts like calcium hydroxide, however these are industrial lubricants more than they are cleaning products. But because they result from the same chemical process they are still referred to as ‘soaps.’
Ammonia and bleach aren’t alkali salts, and as far as I am aware, the alkali salt is critical to the chemical process and you simply cannot make soap from chemicals which are not alkali salts. You can add them to soaps for the purposes of enhanced disinfectant and whitening properties (though this is not done for body-soap. Ever. Because it results in chemical burns. So unless you need to disinfect a counter top…) but they can’t be used as a lye substitute for the creation of soap as far as I am aware.
I’m only going into detail because I can see soap getting added and there’s really no reason to ignore reality when actual soap creation is pretty simple. Simple enough to happen accidentally, even.
As for illness, as far as I am aware there is no evidence that the rhinovirus etc (cold virus) ‘lies dormant’ in the body until you get cold. There is evidence that cold weather plays a role in the cycle of the virus (though theories suggest it is because people spend more time in close quarters, indoors, during the winter and wet seasons, making for a better environment for virus transmission), but the notion that a lone person could develop a cold on the Space Station after ten months of isolation if he just got cold enough is silly. I will go into more detail in a spoiler since it’s not on-topic, but if you’re interested, it’s in there.
[spoiler]Once you have had a cold and fought it off, you are effectively immune to that cold virus for the rest of your life - that is the purpose of antibodies. However, this doesn’t prevent you from transmitting the virus to someone else, at which point it will reproduce and evolve and eventually come back as a virus different enough to confound your antibodies. And so the cycle repeats. However the critical thing here is that the virus needs to be actively attacking cells in a body in order to reproduce and evolve. That is by definition the only way a virus can replicate. Once your body develops antibodies for a virus, there is no way that virus can infect you again unless it leaves your body and its great-great-great grand-kids come knocking next year with new tricks. The same goes for the flu.
More on illness, other cultures, etc; the reason we more easily get sick when visiting other parts of the world is that we are immersing ourselves in an environment full of common viruses that our body has not had an opportunity to develop antibodies for. But that still requires people. You will not get communal diseases if you travel to Alaska and live in the woods by yourself. You get communal diseases from people. Sanitation is an attempt to mitigate what is normally a perfect environment for communal diseases, and reduce the transmission of them. We wash our hands to prevent the transmission of viruses to or from other people. We clean our surgical implements to prevent the transmission of viruses and bacteria to or from other patients. We flush our sewage into treatment facilities to prevent the transmission of viruses (and bacteria) from our waste products to and from other people.
All of these things require people.
It is perfectly possible to get bacterial infections from the environment in the absence of people - fecal coliforms, tetanus, certain animal-born infections and viruses (H1N1 as a famous example). However, the most common illnesses in human society cannot exist in the absence of us. They are viruses which require human populations to exist. They cannot live or reproduce outside of the human body. Therefore you cannot get them if you aren’t around other people. They aren’t floating around in the wilderness or waiting in your body for a moment to strike. Human bodies are the factories for these diseases. If they are floating around in the air it is because an infected person has been there recently, and for no other reason. If they are sitting on a watermelon it is because an infected person touched it.
And for the record, the bacteria on your hands, in your mouth, in your nose, and in your digestive system is constantly fighting off dangerous bacteria. You only notice it when there is too much dangerous bacteria to fight off, and you get sick. And that is when antibodies take over. Without a healthy internal biotic population you would basically not survive long at all. They do a lot of the heavy lifting where it comes to preventing infection. There have been plenty of studies showing how the overuse of disinfectant in ordinary day-to-day life can lead to more and worse illness, for a couple of reasons. One, it leaves the hands barren of harmless bacteria which would immediately attack dangerous bacteria on contact, meaning that dangerous bacteria survives longer on the skin and has a higher chance of reaching the mouth or nose. And two, because small amounts of virus and bacteria being introduced to the body primes our immune system to fight them off more effectively. If you deny your immune system these primers then it is starting from scratch every time a new virus/bacteria is introduced to the body.[/spoiler]
That all said, however; as I stated earlier, I’m not necessarily opposed to the idea that these viruses are getting along just fine in the new zombie population. As long as that is the vector through which the cold and flu viruses are being transmitted, and we aren’t pretending that these viruses are just chilling out on toasters waiting for us to touch them.
As for drugs leading to poor health; this is another misconception. I have a university textbook (‘Drugs and Behavior: An Introduction to Behavioral Pharmacology’ by William A. McKim) right in front of me that says, and I quote;
‘Surprisingly few medical problems arise as a direct result of chronic heroin use.’ It goes on to list just two things; constipation and higher risk of bladder cancer. Not exactly the ‘junkie’ stereotype imagined. As I said before, the ‘junkie’ stereotype is generally not a result of chronic heroin use so much as chronic use often leads to behavioral changes that result in secondary harm. Mostly in the form of poor hygiene, lack of sleep, and diet.
There are drugs which do have chronic longterm health effects (cocaine taken regularly in high doses can result in what amounts to paranoid psychosis until the cycle is broken, which actually would make for an interesting system in-game), but they are typically not what you’d expect. The vast majority of health and psychiatric problems commonly associated with chronic drug use comes down to two things: a lack of a healthy diet, and a lack of sleep.
If someone is interested in having a conversation about the accute and chronic effects of drugs I am happy to oblige; I have (an academic) background in drugs and psychopharmacology. I’m not an expert but I’ve done the courses and I have the texts available. And it certainly interests me.
TLDR; let’s be careful to understand how things work in reality before we start making suggestions for how they ought to work in a game. Although games don’t have to mimic reality, reality is always a good place to start in games which are at least partially simulation. Chemistry and medicine will tell you how this stuff works if you look it up. Me, I’m in favor of relatively simple systems which more or less achieve the same results you’d expect in reality, except in cases where the fantastic is implied (like zombies etc).