Walmart cannot and will not fire you just for being sick and if you're actually sick and need time off work at Walmart then you're allowed time off of work for those sick days.
However if you call in sick too often then you could be fired from Walmart so only call in sick if you absolutely cannot make it to work or would not be able to perform any actual work productively.
For example if you're having the flu and are too weak to barely stand or concentrate then call in sick but if you're just having a cold and are not feeling weak or fatigued then go ahead and go to work.
There were many times that I went into work at Walmart even with the flu and cold because I needed the money and I also didn't want to use too many sick days and risk getting fired.
But if I felt way too weak or fatigued and could barely move around then I would call in sick and stay home.