根据汉语提示写单词。

1．How many _________ (国家) are there in the world?

2．This is the _________ (开始) of our tour.

3．Children should _________ (睡觉) for more than eight hours every day.

4．The girl is _________ (从未) late for school.

5．I think Beijing Duck is very _________ (美味的).

细菌的感染途径一般分为（）、（）和（）。