Webster's New World Medical Dictionary

Hct
Hematocrit.
HCV
Hepatitis C virus. See 
hepatitis C.
HDL
High density lipoprotein. See 
HDL
cholesterol.
HDL cholesterol
High density lipoprotein cho-
lesterol. Lipoproteins, which are combinations of
fats (lipids) and proteins, are the form in which
lipids are transported in the blood. HDLs transport
cholesterol from the tissues of the body to the liver,
so the cholesterol can be eliminated in the bile. HDL
cholesterol is therefore considered the “good” cho-
lesterol: The higher the HDL cholesterol level, the
lower the risk of coronary artery disease.The aver-
age man has an HDL cholesterol level of 40 to 50
mg/dL. In the average woman, HDL levels range from
50 to 60 mg/dL. An HDL cholesterol of 60 mg/dL or
higher gives some protection against heart disease.
Regular aerobic exercise, loss of excess weight (fat),
and cessation of cigarette smoking increase HDL
cholesterol levels. When lifestyle modifications are
insufficient, medications can be used.
HDV
Hepatitis D virus. See 
hepatitis D.
head bones
See 
bones of the head.
head lice
Pediculus humanus capitis, parasitic
insects found on the human scalp. They lay eggs on
the hair shaft close to the root, and hatched lice stay
mostly on the scalp. Head lice infection is very com-
mon and easily acquired by coming in close contact
with someone who has head lice, infested clothing,
or infested belongings. Preschool and elementary
school children and their families are infested most
often. Symptoms of head lice infestation include a
tickling feeling of something moving in the hair,
itching caused by an allergic reaction to the bites,
irritability, and sores on the head caused by scratch-
ing. Although lice are very small, they can be seen
on the scalp when they move. The eggs (nits) are
easily seen on hair shafts. Treatment involves a com-
bination of topical insecticidal medication and man-
ual removal of all nits with a lice comb or the
fingers.