Hello XXXXXXX,

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To further answer your specific questions, it will be depending on the period of onset (duration of rash), size, distribution (clusters, rings, scattered), which is the primary lesions involved (papules, macules, pustules, nodules etc), surface consistency and concours (smooth, clearly demarcated margins etc) and if the rash is itchy or not. A purplish rash on the neck is not necessarily contagious (and if that is the case, then the patient will also testify that another person had the rash or there is a notion of contagiousness). Now such a symptom as unusual purplish rash on neck and arms is unspecific without the above characteristics present and without a doctor or dermatologist having a look at the rash will make determining the cause difficult.
In general, before an unknown rash, it is best to avoid contact with the patient untill a dermatologist is consulted (specific cause established). After which specific treatment can be established so that even if it is a contagious rash it can be treated before affecting another person.
Typically purplish rashes could be due to multiple purpura (which could be due to a local skin infection or an autoimmune diseases or other causes/systemic diseases), dermatomyositis (inflammation of skin and underlying muscles/rare connective tissue diseases), parasitic bites etc.
The potential hazard? Depends on the specific cause of rash (surely from description it might not necessarily be contagious)
The risk to yourself, also depends on specific cause.
Risk to others, also depends on specific cause.Being contagious is the most likely risk.
How to minimize harm to yourself is visiting a doctor as soon as possible to confirm cause and treat, then others will be equally saved.

Hope this helps and let me know if you got any further questions.
Dr. Bernard

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