Sounds like you got some fungus in your shoes you need gone. The reason you have fungus shoes doesn’t matter but we are going to make sure it gets killed.
If you have bought a second hand pair of shoes you may want to do this even if you don’t think they have fungus.
Main causes of fungus / bacteria and things alike are simply from providing a warm, dark, damp environment for nasty things to grow.
Your sweaty feet in your dark, damp shoes is what I am talking about. Other causes can be from improperly storing your shoes for an extended period of time.
Drying your shoes
First and foremost you will want to make sure your shoes are completely dry. If you have fungus shoes I’m assuming they are not the driest.
To make sure your shoes are dry you can leave them out in the sun for a day or so, make sure to clean off your shoes first with some water as this will just make things easier.
Use a rag to initially soak up as much moisture out of your shoes as possible.
When you set your shoes in the sun you can stuff newspapers into the shoes to act as a wicking agent and it will help soak up some of the moisture. Make sure the paper you use is newspaper material and not that plastic paper stuff.
A windy day is a plus for speeding up the drying process, alternatively you can place a fan blowing on the damp shoes.
Once your shoes are absolutely as dry as your can get them you can proceed.
Vinegar / Baking Soda
Vinegar is great for many things, including getting rid of fungus. Vinegar is a type of acid, which does work to bacteria.
After your shoes are dry of moisture, generously apply the vinegar / baking soda solution to the insides of the shoes. Use an old tooth brush or any brush to scrub in the solution to the material.
If you are working with nice leather shoes start with a very diluted solution and add more from there.
Once the shoe has been scrubbed down, let it sit for a few hours so that the solution can do its job. Afterwards rinse off the mix and let dry again then you are good to go!
Rubbing alcohol can be substituted in place of vinegar also.
Fungus builds up when it is in a damp environment so it is important that you do not leave your shoes wet for an extended period of time.
After you get done sweating or doing activities you should allow your shoes to fully dry out, you dont want to put away sweaty shoes.
This may come as an obviously to most people but it isn’t very hygienic to wear socks for more than one day. Avoid wearing your socks for two or more days in a row.
The more time you leave your shoes get dry and damp the more time fungus has to grow.