I suggest refraining from using untethered jailbreak since you'll have to jailbreak your device again every single time it shuts off completely plus it only lasts for 7 days at max.
If you're really desperate you could try but I always waited for the tethered jailbreaks so I don't have to fix it up every week.
About malware or viruses, just don't download things you don't trust or know what they are.
Cydia is overall quite safe though unless you start adding unknown repositories