using wireless internet without security

Some families in my neighbourhood are using wireless internet without any
security.



I know that using security (password or Mac-address filtering) is often
advisable, but I would like to know better what risks are involved by having
an open wireless network:

1) Can virus spread across a wireless network between computers which are
only sharing the internet connection?



2) Is it possible for users sharing an internet connection to gain access to
files on other computers sharing the connection?



Thank you

Erik




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