equally, equitably, fairly, humanely, kindly, leniently, sympathetically, well
abominably, badly, harshly, roughly, shabbily, unfairly, unjustly
They treat their animals quite badly.

seriously
These allegations are being treated very seriously indeed.

accordingly
He is guilty and should be treated accordingly.

in the same way
differently
separately

tend to
Parents still tend to treat boys differently from girls.

as
the tendency to treat older people as helpless

like
Don't treat me like a child!

with
He treated the idea with suspicion.

deserve to be treated
They deserve to be treated with patience and respect.

easily
appropriately, properly
effectively, successfully
medically, surgically

be difficult to
use sth to

for
She was treated for cuts and bruises.

with
We can treat this condition successfully with antibiotics.

be effective in treating sth
The drug is effective at treating depression.

chemically
Chemically treated hair can become dry and brittle.

for
You need to treat this wood for woodworm.

with
The timber has been treated with chemicals to preserve it.

Treat is used with these nouns as the subject:
drug, staff, vet, veterinarian

Treat is used with these nouns as the object:
abscess, acne, addiction, ailment, alcoholism, allergy, arthritis, asthma, burn, cancer, colic, condition, constipation, corn, depression, disease, disorder, HIV, illness, impotence, incontinence, indigestion, infection, infertility, injury, leather, malaria, matter, migraine, pain, patient, phobia, schizophrenia, sewage, sick, sickness, slave, subject, survivor, symptom, syndrome, timber, tumour, victim, waste, wound