There was a famous case of an Internet worm virus in the mid 80's unleashed by a Cornell graduate student. Email systems have since been redesigned and a repeat of that kind of infection of email servers is considered very unlikely. Simple reading of email can not transmit viruses. This hoax has cost more lost productivity by people questioning its existence, calling and asking about it, and preparing for it than any real virus. You can get a virus by executing a program attached to an email message.
Email attachments can contain executable binary files.
Executable binary files can contain viruses. Such incidents are extremely rare. Major software companies are very careful about their media. PCs are more susceptible than Macs, which are more susceptible than UNIX machines. Virus writers want to damage files.