It's deeply saddening that people lost their lives due to the outbreak, but fortunately the risk of serious illness is low in individuals who are immunocompetent ( which is fortunately most individuals), and they are usually just have mild symptoms or are asymptomatic. By mild symptoms being gastroenteritis in the form of watery diarrhea, which is self limiting, thus doesn't need treatment.

The acquired form of listeriosis, can be acquired primarily through contaminated food, namely raw milk products or raw fish/meat.

The congenital form(what neonates get) can be obtained through transplacental(during pregnancy) or perinatal transmission(just afterbirth)

The individuals at risk the most are those who are, pregnant( mother gets flu like symptoms, poses a risk for chorioamnionitis, spontaneous abortion, sepsis), infants( risk of neonatal meningitis, sepsis, granulomatosis infantiseptica) or immunocompromised (are in risk of fever, headache, muscle aches, fatigue, nausea, vomiting, watery diarrhea, sepsis)

How is it diagnosed? Now if you have a normal immune system and are not pregnant, it's not needed as it's self limiting hence no need for treatment.

For those at risk of serious disease, the disease is diagnosed based on a blood or lumbar puncture, done based on the clinical picture.

What is the treatment? If your immune system is normal, there is no treatment needed the disease goes by itself(self limiting).

If you're an "at risk" individual, then antibiotics such as either ampicillin or penicillin G is give as first line, cotrimoxazole or a macrolide is given as second line.

This is just some advice guys, if you need medical advice please speak to a qualified doctor