In someone who is healthy, the spleen is usually small enough that it can't be felt when you press on the abdomen.   Certain diseases can cause the spleen to swell to several times its normal size. Most commonly, this is due to a viral infection, such as mononucleosis.*

In some cases, more serious diseases such as cancer can cause the spleen to expand.  Doctors usually tell someone with an enlarged spleen to avoid contact sports like football for a while, because a swollen spleen is vulnerable to rupturing (bursting). If the spleen ruptures, it can cause a huge amount of blood to be lost.*