Email spam and viruses

Recently I have read and a news announcement that there was revealed that there is possibilitie to get a spam letter already with a viruse attacht to it. I think this is already accouple of days old news.
Basicaly spam letters is trying to sell something to its readers, for example, questionable pharmaceutical products. If spam email is opened that might be the reason why next day email inbox just flooded with such letters. Why not to try to sell you something else if you have opened one spam letter. So this way it is possible to become a target for various spamers.
Now, why to send to letters, one with spam and another one with viruse if it is possible to do these things in one letter.
Reading spam letters might cause some serious problems. I guess there is no need to explain how viruses might be dangerous.
Accourding to various web sources the global ratio of spam in email traffic from bad sources was 76.1 per cent and it is higer numbers that some time before.