Answer: If you use other people's toothbrush, razor (especially blades), hairbrush, comb, deodorant, etc., you risk acquiring the germs (normal flora and pathogenic) that person has on their body.
Some diseases or conditions you can "get" (acquire) from using other people's personal hygiene items include:
chronic skin conditions
bacterial skin conditions
any number of bacteria found in the mouth
bacteria from gum disease
yeast in the mouth
nits and lice in the hair
scalp conditions
crabs (genital, from using someone else's razor or comb)
Blood borne disorders from using a used razor
Blood borne disorders from mouth viruses and bacteria
Fungal infections -- including from wearing other people's shoes or flip flops
Skin disorders from using someone's pillow without using a clean pillow protector and pillow case
NOTE: The words condition, disease, disorder all mean different things. Acute is 'now'. Chronic is long term. Many chronic problems become 'life long'.