The Zero Article

with​​ names​​ of people,​​ places, companies, countries, etc.:

Jack lives​​ in London. I am friends with both​​ Mary and Peter.

with names of​​ meals:​​ 

have dinner, have lunch

with the names of​​ some illnesses:

He has​​ high blood​​ pressure.

when talking about​​ travelling:​​ 

go by car/train

with​​ plural nouns​​ when talking in general:

Katie likes exotic​​ animals.

with​​ uncountable nouns​​ when talking in general:

Cheese is made from​​ milk.

when talking about​​ certain places​​ where the​​ function​​ of the place is the most important:

in hospital / at​​ work / in prison / in bed / at school / at university