Nerdtree not recognized as command in VIM (ubuntu)

I have installed pathogen.vim from github into the `.vim/autoload` directory and have edited my .vimrc to include the following lines ``` pathogen#infect() pathogen#helptags() autocmd vimenter * NERDtree ``` I have installed the latest version of nerdtree from GitHub > scrooloose/nerdtree into my `.vim/bundle` directory despite this, vim tells me that NERDtree is not a recognized command. I am now at a loss. Any help would be very much appreciated. Steve Kane