
behandeln
to treat
To deal with a person or thing in a particular way.
Man sollte alle Menschen mit Respekt behandeln.
One should treat all people with respect.
to treat / to deal with
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to treat
To deal with a person or thing in a particular way.
Man sollte alle Menschen mit Respekt behandeln.
One should treat all people with respect.

to treat
To provide medical care for an illness or injury in order to heal or relieve it.
Der Arzt behandelt die Infektion mit einem Antibiotikum.
The doctor treats the infection with an antibiotic.

to deal with
To concern oneself with a particular topic; to make it the subject of a discussion, text, or work of art.
Der Film behandelt das Leben einer berühmten Wissenschaftlerin.
The film deals with the life of a famous scientist.

to treat
To work on a material or surface with a substance in order to protect, clean, or change it.
Das Holz für die Terrasse wurde speziell behandelt, damit es wetterfest ist.
The wood for the terrace was specially treated so that it is weatherproof.
'Behandeln' (to treat / to deal with) is an inseparable verb because it begins with the prefix 'be-'. The prefix makes the verb 'handeln' (to act / to trade) transitive, which means that 'behandeln' always requires an accusative object.
Ich behandle dieses Thema in meinem nächsten Vortrag.
I deal with this topic in my next lecture.
The exact meaning of 'behandeln' (to treat / to deal with) depends strongly on the object. You treat people, illnesses, topics, or materials. The context makes clear which meaning is intended.
Der Chef behandelt seine Mitarbeiter fair.
The boss treats his employees fairly.
When the object is a person, 'behandeln' (to treat) describes the way they are dealt with. Often an adverb or a 'wie' (like/as) clause is added to describe the manner.
Er behandelt seinen Hund wie ein Familienmitglied.
He treats his dog like a family member.
In a medical context, 'behandeln' (to treat) is often used with the preposition 'mit' (with) to name the medication or method. The object is the illness or the patient.
Die Ärztin behandelt die Grippe mit Ruhe und viel Tee.
The doctor treats the flu with rest and lots of tea.
When topics are involved, the subject is often a book, a film, or a lecture. The verb then describes which topic is dealt with in it.
Dieses Buch behandelt die Geschichte des 20. Jahrhunderts.
This book deals with the history of the 20th century.
to act / trade
Both words can identify the subject of a text or film. „Handeln von“ is used with „von“ and describes the content, while „behandeln“ takes the topic as an accusative object and emphasizes that the text deals with it. So pay attention to the complement: „von etwas handeln“, but „etwas behandeln“.

Der Film behandelt die schwierige Suche der Familie nach einem neuen Zuhause.
The film deals with the family’s difficult search for a new home.
Der Film handelt von einer Familie, die ihr Zuhause verlassen muss.
The film is about a family that has to leave its home.